Add feature flag around the presence indicator in room list

This adds a feature flag (disabled by default for now) around the room list
presence indicator, as it still needs some tweaking before it's ready for
production use.

Part of https://github.com/vector-im/riot-web/issues/11799

src/settings/Settings.js

@@ -134,6 +134,12 @@ export const SETTINGS = {
         supportedLevels: LEVELS_FEATURE,
         default: false,
     },
+    "feature_presence_in_room_list": {
+        isFeature: true,
+        displayName: _td("Show a presence dot next to DMs in the room list"),
+        supportedLevels: LEVELS_FEATURE,
+        default: false,
+    },
     "mjolnirRooms": {
         supportedLevels: ['account'],
         default: [],