This adds support for course staff masquerading as any other user when
viewing their own course, via user={username} querystring parameter.
Rules:
* Anonymous users are repesented by a blank "user" value.
* If there is no "user" parameter at all, render for the user making the
request.
* Global staff can view any course as any user.
* Course staff and instructors can view their own course as any user
(including other staff, the anonymous user, or users not enrolled in
their course).
This commit supports TNL-8330 (switching the frontend-app-learning MFE
to use the Learning Sequences API instead if Course Blocks).
caf75dc0d9