feat(App): move participant leaving logic to base/participants

react/features/app/components/AbstractApp.js

 import Thunk from 'redux-thunk';
 
 import { i18next } from '../../base/i18n';
-import { localParticipantLeft } from '../../base/participants';
 import {
     MiddlewareRegistry,
     ReducerRegistry,
     }
 
     /**
-     * Init lib-jitsi-meet and create local participant when component is going
-     * to be mounted.
+     * Initializes the app.
      *
      * @inheritdoc
      */
     }
 
     /**
-     * Dispose lib-jitsi-meet and remove local participant when component is
-     * going to be unmounted.
+     * De-initializes the app.
      *
      * @inheritdoc
      */
     componentWillUnmount() {
         const { dispatch } = this._getStore();
 
-        dispatch(localParticipantLeft());
-
         dispatch(appWillUnmount(this));
     }
 

react/features/base/participants/middleware.js

 import {
     localParticipantIdChanged,
     localParticipantJoined,
+    localParticipantLeft,
     participantLeft,
     participantUpdated
 } from './actions';
 
     case APP_WILL_UNMOUNT:
         _unregisterSounds(store);
-        break;
+
+        return _localParticipantLeft(store, next, action);
 
     case CONFERENCE_WILL_JOIN:
         store.dispatch(localParticipantIdChanged(action.conference.myUserId()));
     return result;
 }
 
+/**
+ * Signals that the local participant has left.
+ *
+ * @param {Store} store - The redux store.
+ * @param {Dispatch} next - The redux {@code dispatch} function to dispatch the
+ * specified {@code action} into the specified {@code store}.
+ * @param {Action} action - The redux action which is being dispatched in the
+ * specified {@code store}.
+ * @private
+ * @returns {Object} The value returned by {@code next(action)}.
+ */
+function _localParticipantLeft({ dispatch }, next, action) {
+    const result = next(action);
+
+    dispatch(localParticipantLeft());
+
+    return result;
+}
+
 /**
  * Plays sounds when participants join/leave conference.
  *