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

Requires https://github.com/HabitRPG/habitrpg-shared/pull/304

Implements SabreCat's idea for a workaround for the To-Do sorting bug: https://github.com/HabitRPG/habitrpg/issues/2606#issuecomment-53584917
I.e., this PR causes a To-Do to be moved to the bottom of the list when the To-Do is marked as completed.


This PR also adds a "Push task to bottom" button, but it's commented out until we decide if we want it. I would like to use the button, but I'm not sure it's worth the extra icon clutter. Perhaps we can enable it in future when we've hidden some icons by default until the user hovers over the task.