Allow close reasons to have a field for freeform text that a closer can use

[Oaphi]

Per request in Discord - this seems to be unanimously supported by us, and given that it'd be a configuration option (I believe it should be disabled by default), I don't see any downsides (apart from potential for abuse, but closure is a highly visible action and can only be done by users with the ability, so it's unlikely to be misused - and even if it will be, can be easily dealt with on a case by case basis).

Distantly related: #911

[cellio]

Let's make sure that people who can close/reopen can also hide a problematic entry here if needed. They shouldn't need to reopen and then reclose, leaving a confusing post history and changing the attribution for the closer.

(I originally wrote "delete" instead of "hide", but we probably want moderators to be aware that it happened. Not sure of the best way to do this in the UI.)

[Oaphi]

(I originally wrote "delete" instead of "hide", but we probably want moderators to be aware that it happened. Not sure of the best way to do this in the UI.)

I beleive deletion will be fine here too - we can just add a moderator audit log with the content so as they can restore should the need arise

[cellio]

It might be time to think about a moderator-level audit log (let's call it something different to avoid confusion) and move appropriate items from the admin audit log to there. Deleting a close reason will be rare so doesn't seem to rate its own tool. On the other hand, we already record some rare things in the admin log that maybe should be be available to moderators, like tag merge/rename/nuke, comment delete/undelete, and ability grant/revoke/suspend. (This is off the top of my head; I haven't reviewed everything we include in the audit log now, some of which is definitely admin-only.)