Use upstream slack-go/slack again (#1018)

vendor/github.com/slack-go/slack/slackutilsx/slackutilsx.go

@@ -0,0 +1,62 @@
+// Package slackutilsx is a utility package that doesn't promise API stability.
+// its for experimental functionality and utilities.
+package slackutilsx
+
+import (
+	"strings"
+	"unicode/utf8"
+)
+
+// ChannelType the type of channel based on the channelID
+type ChannelType int
+
+func (t ChannelType) String() string {
+	switch t {
+	case CTypeDM:
+		return "Direct"
+	case CTypeGroup:
+		return "Group"
+	case CTypeChannel:
+		return "Channel"
+	default:
+		return "Unknown"
+	}
+}
+
+const (
+	// CTypeUnknown represents channels we cannot properly detect.
+	CTypeUnknown ChannelType = iota
+	// CTypeDM is a private channel between two slack users.
+	CTypeDM
+	// CTypeGroup is a group channel.
+	CTypeGroup
+	// CTypeChannel is a public channel.
+	CTypeChannel
+)
+
+// DetectChannelType converts a channelID to a ChannelType.
+// channelID must not be empty. However, if it is empty, the channel type will default to Unknown.
+func DetectChannelType(channelID string) ChannelType {
+	// intentionally ignore the error and just default to CTypeUnknown
+	switch r, _ := utf8.DecodeRuneInString(channelID); r {
+	case 'C':
+		return CTypeChannel
+	case 'G':
+		return CTypeGroup
+	case 'D':
+		return CTypeDM
+	default:
+		return CTypeUnknown
+	}
+}
+
+// EscapeMessage text
+func EscapeMessage(message string) string {
+	replacer := strings.NewReplacer("&", "&amp;", "<", "&lt;", ">", "&gt;")
+	return replacer.Replace(message)
+}
+
+// Retryable errors return true.
+type Retryable interface {
+	Retryable() bool
+}