Simplify lookup for events where we don't use QR codes

[evan10s]

Some of our smaller events don't use QR codes and our HackGT/checkin-android app for checking in participants. We currently look people up by their name or registration email, but this causes a number of issues since this goes off of the name on the registration account rather than the event application.

As an alternative, registration should support generating short-codes (e.g., "4JEF" and so on) that we can use to find users quickly in registration.

Related to HackGT/registration#270