Make it easier to delete tags

[cellio]

Sort of from https://meta.codidact.com/comments/thread/4917.

It's not currently possible to delete tags; you have to instead merge them into something else (which also confuses the edit history on any affected posts). Can we give moderators a way to delete a tag outright?

As a minimal implementation, it's ok to require that the tag have zero uses. In the future it would be nice to be able to delete a tag that's in use so long as it wouldn't leave any questions completely untagged, but that would require more UI tooling (and thought).

If we implement tag synonyms, we will need to distinguish "0 entries because unused" from "0 entries because it's just a pointer to another tag". If synonyms aren't tags themselves and don't show up in the tags list, this concern is mitigated.