Allows remote users to change their username (#27839)

server/platform/services/sharedchannel/sync_recv.go

@@ -212,9 +212,9 @@ func (scs *Service) upsertSyncUser(c request.CTX, user *model.User, channel *mod
 		// save the updated username and email in props
 		user.SetProp(model.UserPropsKeyRemoteUsername, user.Username)
 		user.SetProp(model.UserPropsKeyRemoteEmail, user.Email)
-		// TODO:  MM-59398:  allow patching username with munged, unique name
 
 		patch := &model.UserPatch{
+			Username:  &user.Username,
 			Nickname:  &user.Nickname,
 			FirstName: &user.FirstName,
 			LastName:  &user.LastName,