This change is a follow-up to the chages in PR 8347, which removed the
edX login page from the workflow for a new user. Where previously we
redirected a user to the login page, PR 8347 instead creates a new user
transparently and logs them in.

The initial reason for splitting the LTI view between lti_launch and
lti_run was so that there was a target for the GET request that
followed the login page. Since we no longer use the login page, we
no longer need the second view. We also don't need to store the LTI
parameters in the session any more, since they are not persisting
between calls. This simplifies the view logic significantly.

The other change here is to fetch the LtiConsumer object early in
the view, and pass it to the SignatureValidator and scoring system.
When the views were split, this required multiple DB hits for the
same data; we're now only fetching it once.