Users can request to join an Organization's CCLA

:fork_and_knife: 

When users who are not authorized to contribute under a given CCLA view the list of contributors who _are_ authorized (`/ccla-signatures/:ccla_signature_id/contributors`) they will see a button which, when clicked, sends an email on their behalf to the organization's admins requesting to join the CCLA. From the email, admins can either Accept or Decline the invitation.

Right now, there is no behavior behind the Accept or Decline action, so the whole feature is disabled until the `JOIN_CCLA_ENABLED` environment variable is set in production.
Do we want any validations on this model so we don't end up with records missing an `organization`, `ccla_signature` or `user`?