This change cleans up the work in progress request at #8176

This is an initial authentication implementation that allows LTI users to
log in transparently to edX. The behavior is driven by pilot users at
Harvard; this was the most requested feature.

The patch creates a new database model that maps users' LTI identifiers
to newly-created edX accounts. If an LTI launch comes in with a user_id
field that is not in the database, a new edX account is created with a
random user name and password. This account is then stored in the
database, so that it is permanently associated with the LTI user ID.

This patch takes a simplistic approach to session management. If a user
is logged in with a different account when they perform an LTI launch,
they will be logged out and then re-logged in using their LTI account.

In order to keep the patch simple, I have split out some refactoring
that needs to be done into a separate branch that I'll post once this
has been merged. Since we no longer redirect to the login page, we don't
need to maintain two separate LTI endpoints (one for the LTI launch and
one for authenticated users), or deal with the session management that
requires. There are also multiple fetches of the LtiConsumer object
(one in the view, one in the signature validation) that the later
patch will consolidate into one.

This branch fixes the previous conflicts with the test refactoring
carried out in PR 8240.