Add assertion that session doesn't contain element

Add a method assert_session_does_not_contain that asserts that a
KeyError is raised for a given key when accessing the session.

Change the existing assert_session_contains method so that it
is the only thing checking for a KeyError and raising an
AssertionError because of that. This is because otherwise
we would be asserting that an AssertionError is raised, which
is confusing.
Implement dashboard creation form

This commit introduces a form that will POST to Stagecraft to
create a dashboard.

It makes use of the session to store the form if it fails validation,
and then presents it back to the user.