Fix some miscellaneous weirdness around push notifications

For some reason, I *stopped* the timer if we get a push message - which
makes *no* sense. It meant that NeoChat kept running in the background
for basically no reason.

We can also make the NotificationsManager::postPushNotification function
static and avoid creating some singletons.

(cherry picked from commit 94ea1305b2)

src/app/notificationsmanager.h

@@ -53,7 +53,7 @@ public:
     /**
      * @brief Display a native notification for the given push notification.
      */
-    void postPushNotification(const QByteArray &message);
+    static void postPushNotification(const QByteArray &message);
 
     /**
      * @brief Handle the notifications for the given connection.