Correct behavior on updating user slug.

No Issue
* Prevent save while slug is being checked.
* If new slug is empty or all whitespace, reset to previous value.
* If new slug is the same as existing slug except for an increment
  value (e.g. ghost-user-2), use the original slug.
* If the slug has changed, change the URL path to reflect the
  change so that the browser refresh and back button still work.
Just return the promise here (once you update the outer promise to update lastPromise).