Sign-in sheet in progress:
1. Added sign-in code to RecordParticipationActivity
2. Also fixed RecordParticipationActivity's back stack to conform to
Android's navigation guidelines. If you come into this activity from a
notification (as opposed to, say, the "Pending
Participation" activity), clicking on the back button will take you back
to the app's main screen instead of dropping you out of the app
altogether.