Add formal support for spinners on buttons.
Review Request #11046 — Created June 17, 2020 and submitted
This makes use of the new spinner support in Djblets, adding the ability
to show a spinner on a button using styling appropriate for the state
the button is in.
It also fixes up some styling for legacy
Tested this with the Enable/Disable buttons on the new Extensions page.
Verified the spinner was styled correctly for all states.