Fold branch.sql into schema.sql

Per comment on #1547 this didn't actually get run in production, but is
needed for the schema to be built properly in dev. In production we had
a thread that dropped and recreated these tables (which #1547 changed).
Merge branch 'master' into masspay
This adds a participants.paypal_email field

The purpose of this field is to record a PayPal email address for people
whom we're paying out via MassPay. Since MassPay isn't intended as a
long-term option, I'm proposing that we don't need a UI for this field.
I'll update it manually via psql as needed. Generally I hear from people
on support@gittip.com. I confirm the email address manually by asking
for the first eight digits of their API key.