Per #28, Gittip is now opt-in. This commit is the first step towards
implementing that. Next steps will be updating the leaderboard to
differentiate unclaimed accounts, and then of course modifying the
billing script to only process tips to claimed accounts.
We now return claimed_time with the query behind payday, and filter on
that when computing the total to charge a tipper's credit card and how
to distribute their gifts. This addresses #28. We also now have that
info (claimed_time) available on the tipper's profile page. We should
indicate on their which of their tips are to people who have claimed
their account and which are not.