Organization admins can revoke CCLA invitations.

This allows good ole organization admins to revoke invitations. If a user accepts a revoked invitation, they are presented with a 404. This _might_ not be what we want to happen, so we should discuss that!

Pivotal Tracker ID: 65497338
https://www.pivotaltracker.com/story/show/65497338

Worth noting: I called the controller action `revoke` instead of `destroy`. I thought it was more intention revealing of what is happening. Is it a Rails sin to not have the destroy action be called `destroy`? This also leads me to question - should the record be destroy or should we just have a flag that sets it to revoked? do we care about having a record of revoked invitations? All of the questions!