Merge pull request #2 from michaelanthonymain/activity_sign-up-confirm-drop-out

Activity sign up, confirmation and drop out
Added validation in the model so a user cannot have two registrations for the same activity.

Added if statement to controller to check if user is already signed up for an activity
  if so, currently sends back signup page, but will need to be redirected elsewhere.
  if not, sends to confirmation page
Merge branch 'master' into activity_sign-up-confirm-drop-out