In impress-2020, we do a big slow query to figure out which users have
been active in trades recently. Now, we cache that timestamp on the
User model.
This won't have any immediate effect; it's to clear the way for Classic
DTI to receive the better trade ratios feature people like from 2020.
I also added some unit testing infra because I finally wanted it! for
all the ways you can trigger this timestamp lol
Note too that this is a bit of an unusually complex migration, but my
hope is that the batching and query structure and such helps it run
surprisingly fast! 🤞
