This adds a new class of push notification, 'session-change'
which is broadcast to logged-in clients when changes to the user's
state, including the list of groups that they are a member of,
changes.

When the user creates or joins a group, a notification is
added to the event queue with an associated user ID
which is then broadcast via the web socket.

On the client side, the streamer service listens for the new
class of notification and triggers an update of the session
state in response.

When the user leaves a group, this may trigger an implicit change
of focus if the user was a member of the group.

T-105