Use is_authenticated in templates

More self documenting
Add is_public instance property and redirects

We need support for creating private, invitation-only instances.
I implemented this by adding a flag to the instance model and adding
logic to the instance_request decorator to validate the user and
redirect to login or a new, top-level "not available" page as needed.
Show instance name on main instance page

Helps with demos and debugging to show more than a grey button on the
main instance page.
Add is_public flag to make_instance test helper

The url tests were failing after I added the private instance feature
because the default value for is_public is False.
Merge pull request #180 from jwalgran/topic/private-instances

Private Instances