Data: Add a view for settings

Add a 'setting' doc type and a view to
get them.
Backlog: Sign out client if signed out on server

If the server reset and our session was lost,
we could appear to be signed in on the client
if our local session had not yet expired.
Admin: Clean up 'users' panel a little

Make the users section layout consistent with
the groups section, or at least less irksome.
Admin: Update settings

We can now update settings that appear
in the publicly visible /data/settings list,
via the admin page.
Settings: Make a demo user when appropriate

Make a demo user when demo mode is turned on,
and remove the demo user when it is turned off.