This contains the various fixes that needed to be made in order to get
accurate statistics.
Most notably, the active_users_between function in the previous
version of zerver/lib/statistics.py was broken for end dates in the
past, because it used the UserActivity table to get its data -- so in
fact it really was querying "users last active between".
This commit isn't super clean, but I figure we're probably better off
having our latest code for historical usage data in git so it doesn't
bitrot and anyone can improve on it.
(imported from commit 24ff2f24a22e5bdc004ea8043d8da12deb97ff2f)