GitFlow release finish: Implement current workflow

This patch implements what is current release finish workflow in Salsita, which
is roughtly:

  1. Merge release into develop and master.
  2. Tag master.
  3. Checkout develop.
  4. Delete all relevant feature branches (both local and remote).
  5. Push all changes upstream.
  6. Close (submit) all relevant review requests in Review Board.
  7. Deliver all PT stories assigned to this particular release.

This patch also includes another major refactoring, but it's still a terrible
mess and it will be necessary to rewrite it as a whole at one point, possibly as
soon as possible.