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
If I attempted to delete a valid participation but which the policy barred me from, what would the response be?