Reimplement breadcrumbs for new room list

This all-new component handles breadcrumbs a bit more smoothly for the app by always listening to changes even if the component isn't present. This allows the breadcrumbs to remain up to date for when the user re-enables breadcrumbs.

The new behaviour is that we turn breadcrumbs on once the user has a room, and we don't turn it back off for them.

This also introduces a new animation which is more stable and not laggy, though instead of sliding the breadcrumbs pop. This might be undesirable - to be reviewed.

src/settings/SettingsStore.js

@@ -181,6 +181,8 @@ export default class SettingsStore {
      * @param {String} roomId The room ID to monitor for changes in. Use null for all rooms.
      */
     static monitorSetting(settingName, roomId) {
+        roomId = roomId || null; // the thing wants null specifically to work, so appease it.
+
         if (!this._monitors[settingName]) this._monitors[settingName] = {};
 
         const registerWatcher = () => {