Add /_/signout to the server

Now that sessions stick around basically forever,
it can be desirable to sometimes sign out.
Add 'sign out' link on the client

Add a 'sign out' link to the bottom of the page,
that is hidden when the notes have the focus,
and signs us out when clicked.
Version 1.0

Notes is in a capable state, with no need for
additional work to be considered complete.