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
What's this action's response if `participation.present?` is false?

This makes me wonder about using a null object pattern in the `participation` private method. Something like:

```ruby
def participation
  @participation ||= Participation.find(params[:id]) || NullParticipation.new
end
```

Where `NullParticipation`, when responded with, renders a 404 or whatever is most appropriate. Thoughts?