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/sharedchannel/sync_send.go

@@ -315,7 +315,7 @@ func (scs *Service) updateForRemote(task syncTask, rc *model.RemoteCluster) erro
 		// All posts were filtered out, meaning no need to send them. Fast forward SharedChannelRemote's NextSyncAt.
 		scs.updateNextSyncForRemote(scr.Id, rc, nextSince)
 
-		// everything was filtered out, nothing to send.
+		// if there are more posts eligible to sync then schedule another sync
 		if repeat {
 			scs.addTask(newSyncTask(task.channelId, task.remoteId, nil))
 		}