Introduce pry-remote, which allows for the use of pry if you don't have the Rails server running in a terminal somewhere.
Make sure ALL invites are removed when the user hits the 'remove' link.

If a user had somehow got two invites (ie. as both a student AND a
coach), the remove method would only attempt to un-attend one of them.
This changeset ensures that we're removing both, and introduces a unit
test for it too.