Fix up "Save" button last-saved tooltip

Fix updating the "Save" button tooltipster-generated tooltips.

* Call 'update' when the last-saved timestamp is updated
* Only update the title attribute if tooltipster is not present, avoiding
  redundant tooltips
* Hide the tooltip when the button is clicked
* Only check if the tooltipster plugin is present once, rather than with
  each call

Closes #841