Remove participant

References:
* https://www.pivotaltracker.com/story/show/78929658
* https://www.pivotaltracker.com/story/show/80376832

Re-implement removing participants to work with the changes made to the participation implementation.

This PR replaces pull request #469 .

--- AC + FF
In this particular case, I believe it would `500` because there is not a `destroy` template and no json was explicitly returned.

I really don't know how this particular case should be handled or what the response should be.  If there is no participation, it could be that it was already deleted, or someone was hacking.  So, we could maybe return a `404`, but without a participant I cannot run it through the `ParticipationPolicy` so it would need handled at a higher level.