Handle double-submitted attendance better

[dehnert]

If somebody comes in and doesn't pay, gate should press the "Present but didn't pay" button to add an attendance record (and the person will be underlined and orange). When they then pay, the natural thing to do is to press "Paid $5 cash", which adds a payment record and an attendance record. This means that they end up with two attendee records, which results in a misleading attendee count on https://squaresdb.mit.edu/gate/signin/ and misleading dance counts on https://squaresdb.mit.edu/gate/voting/.

We should fix this, presumably through one or more of the following:

1. Change the "Paid $5" button to not request an attendee record
2. Make the API silently suppress attendee record creation
3. Delete the attendee records after the fact
4. Ignore duplicate attendee records

Probably one of the first two is preferable.

[dehnert]

Make the API silently suppress attendee record creation

Ultimately, "attended the same dance" is semantically meaningless, so SquaresDB now (commit 15e8054) suppresses multiple in the API, which should largely if not completely resolve this.