PLT-4847 Fix formatting of links imported from Slack. (#4674)

This fixes mailto: links built from email addresses posted to Slack as
described in the ticket, but also fixes any other type of URL that has
been auto-linked in the Slack messages.

api/slackimport.go

@@ -494,6 +494,19 @@ func SlackConvertChannelMentions(channels []SlackChannel, posts map[string][]Sla
 	return posts
 }
 
+func SlackConvertPostsMarkup(posts map[string][]SlackPost) map[string][]SlackPost {
+	// Convert URLs in Slack's format to Markdown format.
+	regex := regexp.MustCompile(`<([^|<>]+)\|([^|<>]+)>`)
+
+	for channelName, channelPosts := range posts {
+		for postIdx, post := range channelPosts {
+			posts[channelName][postIdx].Text = regex.ReplaceAllString(post.Text, "[$2]($1)")
+		}
+	}
+
+	return posts
+}
+
 func SlackImport(fileData multipart.File, fileSize int64, teamID string) (*model.AppError, *bytes.Buffer) {
 	// Create log file
 	log := bytes.NewBufferString(utils.T("api.slackimport.slack_import.log"))
@@ -536,6 +549,7 @@ func SlackImport(fileData multipart.File, fileSize int64, teamID string) (*model
 
 	posts = SlackConvertUserMentions(users, posts)
 	posts = SlackConvertChannelMentions(channels, posts)
+	posts = SlackConvertPostsMarkup(posts)
 
 	addedUsers := SlackAddUsers(teamID, users, log)
 	botUser := SlackAddBotUser(teamID, log)