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

app/session_test.go

@@ -448,7 +448,7 @@ func TestGetRemoteClusterSession(t *testing.T) {
 	rc := model.RemoteCluster{
 		RemoteId:     remoteId,
 		RemoteTeamId: model.NewId(),
-		DisplayName:  "test",
+		Name:         "test",
 		Token:        token,
 		CreatorId:    model.NewId(),
 	}