Guests should belong_to Invitation, not a habtm.
Add course association to menu choice model

Forgot to add the course association into the model when
adding the attribute in an earlier migration.
Whitespace trim.
Correct detection of whether a guest already has choices for this course.
Make finding invitations more intelligent.

If 0, show a helpful "not found" page
  (NB: Don’t think this is a 404)
If 1, just redirect to that invitation
If > 1, show list and get guest to pick.