Organization admins can revoke CCLA invitations.

:fork_and_knife: 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

![screen shot 2014-02-20 at 9 40 56 am](https://f.cloud.github.com/assets/928367/2219684/235b8d9e-9a44-11e3-924e-9d8c13777c87.png)


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 DELETE action be called `destroy`? This also leads me to question - should the record be destroyed or should we just have a flag that gets set to `revoked=true`? Do we care about having a record of revoked invitations? All of the questions!