pjax: Fix state corruption

If a new PJAX request is started before the current PJAX request finishes,
leave the user interface in a consistent state.

  * Finish and re-start the loadingbar animation
  * Ensure the overlay is hidden
  * Cancel all current animations (without using clearTimeout)

This also fixes other nuances such as the help tips not working after the
PJAX ui state was corrupted.