Refactor the alphabetic paginator.

This updates Review Board for the associated Djblets change to refactor the
alphabetic paginator into a subclass.

Testing Done:
- Unit tests all pass
- All button cases manually tested. (Forward, back, first and last arrows, each
  letter).
- Verified that  changes to not affect any other module that inherits from
  datagrid (such as the groups list, or the all review requests list, the
  current 'master' of reviewboard with no changes to the userGrid).

Bugs closed: 829

Reviewed at https://reviews.reviewboard.org/r/6522/