* Only check for conflicts with existing aliases/implications when
requests are created or approved, not when requests are rejected.
* Use `update!(status: "deleted")` instead of `update(status: "deleted")`
so that if rejecting the request fails we fail immediately instead of
continuing on and updating the forum topic.
* Wrap `reject!` and `TagChangeRequestPruner.reject_expired` in
transactions so that if updating either the request or the forum
fails, they both get rolled back.
e3ae87cff7