Fix the "Try it out" button to login as a demo user

This is not the long term solution. Ideally, a login will not be required
to view sessions, and all anonymous users will have access to the demo
sessions.

Fixes #40.
Add a default role to new users to see the demo sessions

This is another short-term fix, until demo sessions can automatically be
accessed by everyone.
Add temporary support for a special role for 'brown.edu' users
Merge branch 'demo_sessions'

* demo_sessions:
  Add temporary support for a special role for 'brown.edu' users
  Add a default role to new users to see the demo sessions
  Fix the "Try it out" button to login as a demo user