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.
Need to grab the promise here and set `lastPromise` right before the end of this func.