user_ips is kinda big, so really we want to add the index in the background
once we're running. Replace the schema delta with one which will do that.
I've done this in a way that's reasonably easy to reuse as there a few other
indexes I need, and I don't suppose they will be the last.
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.
Record the device_id when we add a client ip; it's somewhat redundant as we
could get it via the access_token, but it will make querying rather easier.
