Users can request to join an Organization

:construction:

Users can request to join an organization from each organization's list of contributors (`/ccla-signatures/:id/contributors`). When a user makes such a request, all of the admins of that organization receive an email from which they can either accept or decline the invitation.

I've limited the button placement to only the one view for now because it looks a bit cluttered on the organization index view.

Still to go:

- [ ] JavaScript to prevent duplicate history items
- [ ] an indicator that a request to join is pending
- [ ] a feature flag so that all of this is disabled in production for the time being
- [ ] a hard look at the controller specs