Enable user invites

[hillaryj]

To fully transition from cg-dashboard to the Stratos-based dashboard, we need to enable user invites through the dashboard.

Stratos offers this functionality, which can be enabled by following their guide:
https://github.com/cloudfoundry/stratos/blob/master/docs/invite-user-guide.md

Acceptance Criteria

• Users are able to send invites from the Stratos dashboard

[bengerman13]

I don't understand fully how invites work, but I know that cg-uaa-extras does stuff to finalize user creations when we use the internal IdP, so we may need to coordinate changes over there (or we may not)