add support for pushTaskToBottom to help with To-Do sort order bug

Goes along with another PR that I'll make soon.

Implements SabreCat's idea for a workaround for the To-Do sorting bug: https://github.com/HabitRPG/habitrpg/issues/2606#issuecomment-53584917

Also allows future implementation of a manual pushTaskToBottom button, should we want that.