[flags] complications with deleting event

[luceos]

The Deleting event in the flarum/flags extension was refactored here https://github.com/flarum/flags/pull/35.

As a consequence:

• We cannot listen to the Deleting event and send anything back to the user because the flags for the post haven't been actually deleted yet.
• We cannot use the afterDelete callback from the AbstractModel because deletion is executed from the eloquent relationship, which uses an underlying query Builder object which doesn't fire events.

In addition, the original comment made by Ian (https://github.com/flarum/flags/pull/35#discussion_r563206641) - to me - isn't really, entirely valid. Our flags implementation dismisses all flags from a post when the flag(s) on that post are reviewed; it is one interaction that causes the dismissal of all flags of one post.

As such we need to remedy this by doing:

• Reinstate or move logic from FlagsWillBeDeleted to Deleting.
• Or fire a FlagsHaveBeenDeleted event.
• Use the post as argument in the event, not the flags (which are deleted).

[askvortsov1]

I like:

• Using the post as the argument
• Adding a Deleted event in addition to Deleting