This change fixes the spinners in the config-forms styles to use the font

awesome spinner, and removes some extra styles for the activity indicator which

got copied over from Review Board but really shouldn't have been in djblets at

all (there's no activity indicator in any of the djblets code).
As part of this, I've added a template that loads font-awesome from

cdnjs.cloudflare.com. This works similarly to the way djblets currently loads

various javascript libraries, and is intended to be overridden by templates in

any consuming application.

Verified that the cdnjs font-awesome style was loaded correctly when an

  override was not present, and that overriding the fontawesome.html template

  worked.
Checked that the spinner looked correct in the "API Tokens" page.
Enabled and disabled some extensions and checked that the activity indicator

  wasn't improperly overridden.