Bug fix: missing enrollment.id on urls for "Leave" and "Unsubscribe"
Refactor EnrollmentsController by using load_and_authorize_resource
Add test coverage for agencies unsubscribing users

Fixes bug where enrollments#destroy will assume that the leave action
is for the current user, whereas sometimes campaign owners want to
unsubscribe a user.

We also test the case where a user which does not own a campaign, tries
to unsubscribe a user.