Only show `user_login` in `edit` context

`user_nicename` is publicly exposed, but `user_login` is only used for
logging in.
Sanity check tests
Expose `registered` as `registered_date` for `view` context
Rename `avatar` to `avatar_url` to be more explicit
Merge pull request #701 from WP-API/605-user-fields

Fields cleanup for User controller