…le#24170)

- Created a new "sync debug" pref screen to hold the Fxa, Sync, and Push
  server override prefs.  They were taking a lot of screen space on the
  top-level settings menu as individual items
- Added button on that screen to quit FF which is needed to apply the
  changes.
    - This is definitely not the nicest UI, but hopefully QA can just
      override the prefs once save them in an emulator and never have to
      go back to this screen.
    - I do think this is a nicer UI than before, where FF would quit
      after a change to any of the prefs.  That forces you to restart FF
      3 times if you wanted to override all 3 server URLs.