Fix loading spinner page actions buttons

* Hide the number of conversations when the talk page actions button is
  replaced with a spinner.
* Stop page actions jumping when any of the buttons is replaced with a
  spinner (regression from a7a1ed27).
* Reuse .spinner class from the editor in notifications.

Change-Id: Ib9541cd1237316d0157630c019e5d1362584e50d
Merge "Fix loading spinner page actions buttons"