Fix crash when trying to mark two channels as read quickly

If you are on a slower network and you try to mark two channels as read quickly using the context menu, ruqola often crashes. The reason is simple, by the time the user clicks "mark as read" the second time model has changed and the stored QModelIndex is no longer valid leading to a crash. To fix, use a QPersistentModelIndex

Mark as Read, is just one example. Other actions like Quit Channel etc are also prone to this.