Fix validations on user settings page

Closes #3271
* Change validations on both server and client to allow the
  Website field to be empty or a valid URL.
* Add new schema validation helper isEmptyOrURL.
* Remove duplicate call to UserValidator in the save action
  of the SettingsUser controller.
* User.last_login and User.created_at are already Moment objects
  so Moment#fromNow can be called on them directly.