Add support for markdown to HTML conversion (matrix). Closes #663 (#670)

This uses our own gomatrix lib with the SendHTML function which
adds HTML to formatted_body in matrix.
golang-commonmark is used to convert markdown into valid HTML.

vendor/gitlab.com/golang-commonmark/html/.gitlab-ci.yml

@@ -0,0 +1,16 @@
+image: golang:1.11
+
+stages:
+  - build
+  - test
+
+build:
+  stage: build
+  script:
+    - go build ./...
+
+test:
+  stage: test
+  script:
+    - test -z "$(gofmt -l . | tee /dev/stderr)"
+    - go test ./...

vendor/gitlab.com/golang-commonmark/html/LICENSE

@@ -0,0 +1,10 @@
+Copyright (c) 2015, The Authors
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
+
+1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
+
+2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

vendor/gitlab.com/golang-commonmark/html/README.md

@@ -0,0 +1,8 @@
+html [![License](https://img.shields.io/badge/licence-BSD--2--Clause-blue.svg)](https://opensource.org/licenses/BSD-2-Clause) [![GoDoc](http://godoc.org/gitlab.com/golang-commonmark/html?status.svg)](http://godoc.org/gitlab.com/golang-commonmark/html) [![Pipeline status](https://gitlab.com/golang-commonmark/html/badges/master/pipeline.svg)](https://gitlab.com/golang-commonmark/html/commits/master)
+====
+
+Package html provides functions for escaping/unescaping HTML text and for parsing HTML entities.
+
+## Install
+
+    go get -u gitlab.com/golang-commonmark/html

vendor/gitlab.com/golang-commonmark/html/html.go

@@ -0,0 +1,211 @@
+// Copyright 2015 The Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package html provides functions for escaping/unescaping HTML text and for parsing HTML entities.
+package html
+
+import (
+	"io"
+	"strconv"
+	"strings"
+	"unicode/utf8"
+)
+
+const BadEntity = string(utf8.RuneError)
+
+var htmlEscapeReplacer = strings.NewReplacer(
+	"&", "&",
+	"<", "&lt;",
+	">", "&gt;",
+	`"`, "&quot;",
+)
+
+func EscapeString(s string) string {
+	return htmlEscapeReplacer.Replace(s)
+}
+
+func WriteEscapedString(w io.Writer, s string) error {
+	_, err := htmlEscapeReplacer.WriteString(w, s)
+	return err
+}
+
+func isValidEntityCode(c int64) bool {
+	switch {
+	case !utf8.ValidRune(rune(c)):
+		return false
+
+	// never used
+	case c >= 0xfdd0 && c <= 0xfdef:
+		return false
+	case c&0xffff == 0xffff || c&0xffff == 0xfffe:
+		return false
+	// control codes
+	case c >= 0x00 && c <= 0x08:
+		return false
+	case c == 0x0b:
+		return false
+	case c >= 0x0e && c <= 0x1f:
+		return false
+	case c >= 0x7f && c <= 0x9f:
+		return false
+	}
+
+	return true
+}
+
+func letter(b byte) bool { return b >= 'a' && b <= 'z' || b >= 'A' && b <= 'Z' }
+
+func digit(b byte) bool { return b >= '0' && b <= '9' }
+
+func alphanum(b byte) bool { return letter(b) || digit(b) }
+
+func hexDigit(b byte) bool {
+	return digit(b) || b >= 'a' && b <= 'f' || b >= 'A' && b <= 'F'
+}
+
+func ParseEntity(s string) (string, int) {
+	st := 0
+	var n int
+
+	for i := 1; i < len(s); i++ {
+		b := s[i]
+
+		switch st {
+		case 0: // initial state
+			switch {
+			case b == '#':
+				st = 1
+			case letter(b):
+				n = 1
+				st = 2
+			default:
+				return "", 0
+			}
+
+		case 1: // &#
+			switch {
+			case b == 'x' || b == 'X':
+				st = 3
+			case digit(b):
+				n = 1
+				st = 4
+			default:
+				return "", 0
+			}
+
+		case 2: // &q
+			switch {
+			case alphanum(b):
+				n++
+				if n > 31 {
+					return "", 0
+				}
+			case b == ';':
+				if e, ok := entities[s[i-n:i]]; ok {
+					return e, i + 1
+				}
+				return "", 0
+			default:
+				return "", 0
+			}
+
+		case 3: // &#x
+			switch {
+			case hexDigit(b):
+				n = 1
+				st = 5
+			default:
+				return "", 0
+			}
+
+		case 4: // &#0
+			switch {
+			case digit(b):
+				n++
+				if n > 8 {
+					return "", 0
+				}
+			case b == ';':
+				c, _ := strconv.ParseInt(s[i-n:i], 10, 32)
+				if !isValidEntityCode(c) {
+					return BadEntity, i + 1
+				}
+				return string(rune(c)), i + 1
+			default:
+				return "", 0
+			}
+
+		case 5: // &#x0
+			switch {
+			case hexDigit(b):
+				n++
+				if n > 8 {
+					return "", 0
+				}
+			case b == ';':
+				c, err := strconv.ParseInt(s[i-n:i], 16, 32)
+				if err != nil {
+					return BadEntity, i + 1
+				}
+				if !isValidEntityCode(c) {
+					return BadEntity, i + 1
+				}
+				return string(rune(c)), i + 1
+			default:
+				return "", 0
+			}
+		}
+	}
+
+	return "", 0
+}
+
+func UnescapeString(s string) string {
+	i := strings.IndexByte(s, '&')
+	if i < 0 {
+		return s
+	}
+
+	anyChanges := false
+	var entityStr string
+	var entityLen int
+	for i < len(s) {
+		if s[i] == '&' {
+			entityStr, entityLen = ParseEntity(s[i:])
+			if entityLen > 0 {
+				anyChanges = true
+				break
+			}
+		}
+		i++
+	}
+
+	if !anyChanges {
+		return s
+	}
+
+	buf := make([]byte, len(s)-entityLen+len(entityStr))
+	copy(buf[:i], s)
+	n := copy(buf[i:], entityStr)
+	j := i + n
+	i += entityLen
+	for i < len(s) {
+		b := s[i]
+		if b == '&' {
+			entityStr, entityLen = ParseEntity(s[i:])
+			if entityLen > 0 {
+				n = copy(buf[j:], entityStr)
+				j += n
+				i += entityLen
+				continue
+			}
+		}
+
+		buf[j] = b
+		j++
+		i++
+	}
+
+	return string(buf[:j])
+}