Start the work of clearly distingushing client side types, server side
types and types shared over the API by renaming "AccountSession" and
"Session" names used on the server to "ServerSession".
If a user logs out from a device the expectation should be that device
no longer having any association with the user's account. Any existing
push notifications should thefore be removed on server. For this reason
tie push notifications to a session, and remove them when the session is
deleted.
