add setting to throw new RoomSubList hide behaviour behind an opt-in

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

src/settings/Settings.js

@@ -284,4 +284,9 @@ export const SETTINGS = {
         supportedLevels: ['room-device'],
         default: false,
     },
+    "RoomSubList.showEmpty": {
+        supportedLevels: LEVELS_ACCOUNT_SETTINGS,
+        displayName: _td('Show empty room list headings'),
+        default: true,
+    },
 };