Discussion: flow for a subset of participations

There is a situation when only a subset of students were required to finish a flow individually. In this case, the grading/by-opportunity view should only list that subset of participations, instead of all participations of the course.

The filtering criterion can be roles, or ptags. And I think this can be done through adding a few fields in the GradingOpportunity data model, e.g., `restricted_to_roles_all`, `restricted_roles_any`, `restricted_to_participation_tags_all`, and `restricted_to_participation_tags_any`.