Discussion: Allow team assignment

Team assignment means a group of participations to submit a single assignment, rather than a submission per participation. Currently, my idea is to add a participation_tag to team leaders, who are required to submit to flows with `start` flow rule with `if_has_participation_tags_any` or `if_has_participation_tags_all`.

The problem is in the gradebook: it's difficult to identify which participations are supposed to submit the assignment while others not, at least in the `grading/by-opportunity` view.

My proposal is to add a few fields in the `GradingOpportunity`, which are `restricted_roles_all`, `restricted_roles_any`, `restricted_participation_tags_all`, and `restricted_participation_tags_any`, just like what we've done in the flow desc rules. In this way, we can pre filter which participations were supposed to be included in the gradebook views and the grading ui.

Or is there any other good ideas?