Display username 'System' for system messages

web/react/utils/constants.jsx

@@ -121,6 +121,7 @@ export default {
     POST_DELETED: 'deleted',
     POST_TYPE_JOIN_LEAVE: 'system_join_leave',
     SYSTEM_MESSAGE_PREFIX: 'system_',
+    SYSTEM_MESSAGE_PROFILE_NAME: 'System',
     SYSTEM_MESSAGE_PROFILE_IMAGE: '/static/images/logo_compact.png',
     RESERVED_TEAM_NAMES: [
         'www',