fix schema.sql
fix basic auth

fixes https://app.getsentry.com/gratipay/gratipay-com/group/45029121/
fix tests
show user ID on account page and update API doc in README
make sure we always have a user object in the context

fixes https://app.getsentry.com/gratipay/gratipay-com/group/45029122/
save a little memory
fix potential timing attack
backward-compatibility
don't fail on bad userid