Issue management improvements for admins

[phorcys420]

Currently, you can tag posts with both resolved and unresolved if you're an administrator and that's an issue, I think we should account for the following cases:

• Post has resolved tag and admin adds unresolved tag

  • Revoke resolved tag
  • Unarchive thread if archived
  • Send a message saying thread has been unresolved by X

• Post has unresolved tag and admin adds resolved tag

  • Revoke unresolved tag
  • Send a message saying thread has been resolved by X
  • Archive thread if not archived

• Post has an unresolved/resolved tag and it is removed by an admin

  • Rollback to previous tag

---

Currently waiting for nyxx-discord/nyxx#425 to be resolved.