only consider users MAU after they've been around N days.
This is an alternative implementation to https://github.com/matrix-org/synapse/pull/3739
as suggested by @neilisfragile, which is much simpler as you just hold off adding
users to the MAU table until they've been active for more than N days.
implement a GET /devices endpoint which lists all of the user's devices.
It also returns the last IP where we saw that device, so there is some dancing
to fish that out of the user_ips table.