It turns out that it's more useful to return a null device display name (and
let clients decide how to handle it: eg, falling back to device_id) than using
a constant string like "unknown device".
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.
Add a 'devices' table to the storage, as well as a 'device_id' column to
refresh_tokens.
Allow the client to pass a device_id, and initial_device_display_name, to
/login. If login is successful, then register the device in the devices table
if it wasn't known already. If no device_id was supplied, make one up.
Associate the device_id with the access token and refresh token, so that we can
get at it again later. Ensure that the device_id is copied from the refresh
token to the access_token when the token is refreshed.
