Use mattermost v5 module (#1192)

vendor/github.com/mattermost/ldap/README.md

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+[![GoDoc](https://godoc.org/github.com/go-ldap/ldap?status.svg)](https://godoc.org/github.com/go-ldap/ldap)
+[![Build Status](https://travis-ci.org/go-ldap/ldap.svg)](https://travis-ci.org/go-ldap/ldap)
+
+# Basic LDAP v3 functionality for the GO programming language.
+
+## Features:
+
+ - Connecting to LDAP server (non-TLS, TLS, STARTTLS)
+ - Binding to LDAP server
+ - Searching for entries
+ - Filter Compile / Decompile
+ - Paging Search Results
+ - Modify Requests / Responses
+ - Add Requests / Responses
+ - Delete Requests / Responses
+ - Modify DN Requests / Responses
+
+## Examples:
+
+ - search
+ - modify
+
+## Go Modules:
+
+`go get github.com/go-ldap/ldap/v3`
+
+As go-ldap was v2+ when Go Modules came out, updating to Go Modules would be considered a breaking change.
+
+To maintain backwards compatability, we ultimately decided to use subfolders (as v3 was already a branch).
+Whilst this duplicates the code, we can move toward implementing a backwards-compatible versioning system that allows for code reuse.
+The alternative would be to increment the version number, however we believe that this would confuse users as v3 is in line with LDAPv3 (RFC-4511)
+https://tools.ietf.org/html/rfc4511
+
+
+For more info, please visit the pull request that updated to modules.
+https://github.com/go-ldap/ldap/pull/247
+
+To install with `GOMODULE111=off`, use `go get github.com/go-ldap/ldap`
+https://golang.org/cmd/go/#hdr-Legacy_GOPATH_go_get
+
+As always, we are looking for contributors with great ideas on how to best move forward.
+
+
+## Contributing:
+
+Bug reports and pull requests are welcome!
+
+Before submitting a pull request, please make sure tests and verification scripts pass:
+```
+make all
+```
+
+To set up a pre-push hook to run the tests and verify scripts before pushing:
+```
+ln -s ../../.githooks/pre-push .git/hooks/pre-push
+```
+
+---
+The Go gopher was designed by Renee French. (http://reneefrench.blogspot.com/)
+The design is licensed under the Creative Commons 3.0 Attributions license.
+Read this article for more details: http://blog.golang.org/gopher