Adds field for users to choose when they want their reminder email to be
sent out.
Further improves leaderboard performance.

In particular, makes the leaderboard only depend on whether the user is
anonymous, rather than on the user's specific friends and groups.  In
practice this is a small change since most users have at least
PRIVACY_NORMAL for logged-in users.  Also some more random unrelated
performance tweaks.
Merge branch 'master' of github.com:benjaminjkraft/zscore
Moves leaderboard tags to a separate page.
Start of email reminders; to be continued after more sleep happens.
Merge branch 'master' into groups