- "Associated" issues are issues the user has created + issues that the
  user is assigned to.

- Issues that a user owns are transferred to a "Ghost User" (just a
  regular user with `state = 'ghost'` that is created when
  `User.ghost` is called).

- Issues that a user is assigned to are moved to the "Unassigned" state.

- Fix a spec failure in `profile_spec` — a spec was asserting that when a user
  is deleted, `User.count` decreases by 1. After this change, deleting a user
  creates (potentially) a ghost user, causing `User.count` not to change. The
  spec has been updated to look for the relevant user in the assertion.