New HTML email template

See #3082
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
closes #3197
- added role to user obj (only returned from the user endpoint)
- added `/users/?include=roles` and
`/users/?include=roles,roles.permissions` query parameters
- added and updated tests
Merge pull request #3220 from morficus/resend-user-invite

Wiring up "resend" and "revoke" button on user management screen
Merge pull request #3219 from JohnONolan/email

New HTML email template
Merge pull request #3221 from sebgie/issue#3197

Add role to user object