Discussion: Allow multiple instructors in a course with each instructing a subset of student participations

There were multiple instructors teaching a same course with different set of students, which were supposed to have the same course material and schedules. Due to lacking the ability to maintain the repo for all the other teachers, I hope we can use a same course instance, while filter students for each instructors, i.e., each instructor will see only his/her student in the participation list, as well as gradebook apps. That will relate to the following change to the project:

1. New participation permssion which restrict instructors to see only participations with specific participation_tags, which identify the students he/she teaches, and allow editing this pperm in `edit_participation` view.
1. Add participation_tag when preapproving.
1. Add filtering in participation list view, gradebook related views, and filter flowsessions in grading ui as well.

Any thing else? @inducer