Soft-delete tags

[cellio]

When we delete a tag, it is nuked entirely. This makes post histories confusing, as you can no longer see those changes (so you might see a no-op edit). It also means the tag can be recreated, as there's nothing in the DB about it so it's just another new tag.

Can we soft-delete tags instead? We'd want to show them (with some deleted markup, maybe a strikethrough) in histories but not in other places (tags page, main post/category views), so this would mean deletion checks in those views or some backend filtering (TBD). We'd also need to detect and handle the recreation case. But it would make histories clearer and make recovery possible if something important got nuked in error. That, in turn, could allow us to open up tag editing a little more, so that you don't have to be a moderator or admin to delete a tag.