Convert day in user_statistics to a date column and rekey on (user_id,day)

[liamwhite]

Right now it's an integer: https://github.com/philomena-dev/philomena/blob/192cb727a0fb2fc1a4b347a2af6ae0f58466e8f1/priv/repo/structure.sql#L1817

Conversion to:

alter table user_statistics alter column day drop default;
alter table user_statistics alter column day type date using to_timestamp(day*86400)::date;