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
They were testing email delivery in a client side capybara test and adequately covered in a unit test.  They were also intermittently failing.  See: https://www.pivotaltracker.com/story/show/80376832