Correct behavior on updating user slug.

No Issue
- Defer save until after slug is 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.
- Added tests.
Merge pull request #3766 from jaswilli/user-slug

Correct behavior on updating user slug.