Superuser admin

This is an initial implementation of a distinction between superusers and admins, with admins being in charge of particular events.  Major changes:

  - What 'admin' meant in the past now uses the language 'superuser',
    including in ``conf.json`` -- so configurations need to be updated.
    ``UNHANGOUT_ADMIN_EMAILS`` has changed to ``UNHANGOUT_SUPERUSER_EMAILS``.
    Superusers have permission to every event.
  - A new "Users" tab which is visible only to superusers allows
    granting superuser and admin status for particular users and events.
  - Admins viewing the route `/admin/` will only see the events/sessions
    they are allowed to.

What's missing:

  - The link to `Admin` doesn't show up on the home page for admins --
    it only shows up on pages where there is a ``curEvent`` parameter set.
    Since admin-ship is a property of Event models, it requires
    enumerating every Event to see if they should see an admin link.
    (Insert discussion about relational DB's vs key-value stores here. :))
  - There is no way to set the adminship from a the event admin pages.
    Admin-ness is only settable at ``/admin/users/``, and only by
    superusers.  Should be easy to fix, just need to write the UI.
  - No selenium tests for ``/admin/users/`` frontend yet, though the
    backend POSTable plumbing is thoroughly covered.