MM-32133 shared channel username collisions (#17347)

Support for handling username collisions between remote clusters. Users belonging to remote clusters have their username changed to include the remote name e.g. wiggin becomes wiggin:mattermost.

@mentions are also modified so the munged username is replaced with the original username when the post is sync'd with the remote the user belongs to.

When adding remote users:
- append the remote name to the username with colon separator
- append the remote name to the email address with colon separator
- store the original username and email address in user props
- when resolving @mentions replace with the stored original username

services/remotecluster/sendmsg.go

@@ -84,7 +84,9 @@ func (rcs *Service) sendMsg(task sendMsgTask) {
 	defer func() {
 		if r := recover(); r != nil {
 			rcs.server.GetLogger().Log(mlog.LvlRemoteClusterServiceError, "Remote Cluster sendMsg panic",
-				mlog.String("remote", task.rc.DisplayName), mlog.String("msgId", task.msg.Id), mlog.Any("panic", r))
+				mlog.String("remote", task.rc.DisplayName),
+				mlog.String("msgId", task.msg.Id), mlog.Any("panic", r),
+			)
 		}
 
 		if errResp != nil {