Wiring up "resend" and "revoke" button on user management screen

fixes #3214
- new ```User::resendInvite``` method on the User model encapsulates all logic
- only sending users email address when re-inviting, since the user already exists on the back-end
- ```SettingsUserController::revoke``` calls DELETE on ```/ghost/api/v0.1/users/:user_id```