Fix !channel linking in Slack import. (#4065)

Original version of the patch didn't take into account that, like with
@mentions, there's an "old" and a "new" format in the Slack export files
for channel mentions. This version imports both correctly.

api/slackimport.go

@@ -286,15 +286,20 @@ func SlackConvertUserMentions(users []SlackUser, posts map[string][]SlackPost) m
 }
 
 func SlackConvertChannelMentions(channels []SlackChannel, posts map[string][]SlackPost) map[string][]SlackPost {
-	var channelPatterns = make(map[string]string, len(channels))
+	var regexes = make(map[string]*regexp.Regexp, len(channels))
 	for _, channel := range channels {
-		channelPatterns["!"+channel.Name] = "<#" + channel.Id + ">"
+		r, err := regexp.Compile("<#" + channel.Id + `(\|` + channel.Name + ")?>")
+		if err != nil {
+			l4g.Warn(utils.T("api.slackimport.slack_convert_channel_mentions.compile_regexp_failed.warn"), channel.Id, channel.Name)
+			continue
+		}
+		regexes["!"+channel.Name] = r
 	}
 
 	for channelName, channelPosts := range posts {
 		for postIdx, post := range channelPosts {
-			for channelReplace, channelMatch := range channelPatterns {
-				post.Text = strings.Replace(post.Text, channelMatch, channelReplace, -1)
+			for channelReplace, r := range regexes {
+				post.Text = r.ReplaceAllString(post.Text, channelReplace)
 				posts[channelName][postIdx] = post
 			}
 		}

i18n/en.json

@@ -1511,6 +1511,10 @@
     "id": "api.slackimport.slack_convert_user_mentions.compile_regexp_failed.warn",
     "translation": "Failed to compile the @mention matching regular expression for Slack user {{.UserID}} {{.Username}}"
   },
+  {
+    "id": "api.slackimport.slack_convert_channel_mentions.compile_regexp_failed.warn",
+    "translation": "Failed to compile the !channel matching regular expression for Slack channel {{.ChannelID}} {{.ChannelName}}"
+  },
   {
     "id": "api.slackimport.slack_import.log",
     "translation": "Mattermost Slack Import Log\r\n"