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

fixes #3214
- new ```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
- ```revoke``` calls DELETE on /ghost/api/v0.1/users/:user_id
Merge pull request #3220 from morficus/resend-user-invite

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