Add wysisyg composer (can only send message, enable behind a labs flag)

src/settings/Settings.tsx

@@ -303,6 +303,13 @@ export const SETTINGS: {[setting: string]: ISetting} = {
         },
 
     },
+    "feature_wysiwyg_composer": {
+        isFeature: true,
+        labsGroup: LabGroup.Messaging,
+        displayName: _td("Wysiwyg composer (under active development)"),
+        supportedLevels: LEVELS_FEATURE,
+        default: false,
+    },
     "feature_state_counters": {
         isFeature: true,
         labsGroup: LabGroup.Rooms,