PJAX for the staff control panel

PJAX is an ajax technique which appears to load the entire page and even updates the browser's history (so the back button works); however, the page headers and footers are not fetched and reparsed with each page load. This dramatically speeds up the staff control panel (especially the ticket-view page).

### Outstanding
  * [x] Clear timeouts for hover pop-overs when starting a new pjax request
  * [x] Hide the spinner when using the [back] button
  * [x] Fix external links routed through l.php