Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

This change allows the LTI provider to pass grades back to the campus LMS platform using the
LTI outcome service. For full details of the outcome service, see:
    http://www.imsglobal.org/LTI/v1p1/ltiIMGv1p1.html

In brief, the LTI 1.1 spec defines an outcome service that can be offered by an LTI consumer.
The consumer determines whether a score should be returned (in Canvas, this means that the LTI
tool is used in an assignment, and the launch was performed by a student). If so, it sends
two additional parameters along with the LTI launch:

lis_outcome_service_url: the endpoint for the outcome service on the consumer;
lis_result_sourcedid: a unique identifier for the row in the gradebook (i.e. the tool/student/assignment combination).

The LTI Provider launch view detects the presence of these optional fields, and creates database
records for the specific Outcome Service and for the graded LTI launch. Later, when a score on
edX changes (identified using the signal mechanism from previous LTI Provider pull requests),
a Celery task is launched to pass the score back to the LTI consumer.