Use Slug API endpoint for /settings/users/user

Closes #4350
There are a bunch of subtleties here, and I'm not convinced changing the semantics of the `query` objects and hacking up the Adapter is the best way to go.

*If* we were to go this way, I think the proper way to return this model would be to let `store.find` return an array as expect with this calling signature, and then pull the single user object we want to return out by using something like `results.get('firstObject')`.