Terms Controller Redux

* [x] Restore terms endpoints with tests
* [x] Restore taxonomy endpoints with tests
* [ ] Add create and delete methods to term endpoint.
I'd prefer just passing `$request` in here (don't worry about filter compat, since the name will change to `api_prepare_taxonomy` anyway), and defaulting it to `null` so you don't have to bother passing it in to the function if you don't want to.