This PR fixes grade-visibility behavior in the Progress Tab API when staff
users are masquerading as learners.
Previously, subsection show_grades was computed using the real requester’s
staff access, which could expose grades that the masqueraded learner should not
see.
Now, show_grades respects the masqueraded user’s permissions so the API
response matches the learner view.
Co-authored-by: Peter Pinch <pdpinch@mit.edu>