WIP: admins and superusers
Merge refactor-db-loading
Backend functionality and tests all in place
Initial complete implementation of 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.
  - 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.
  - No selenium tests for `/admin/users/` frontend yet, though the
    backend POSTable plumbing is thoroughly covered.
Merge ytID embed fix
Merge pull request #232 from drewww/superuser-admin

Superuser admin