Determine student absence by looking at the enrollments and grade data

[tomca32]

At the moment, our student_lesson view does not know if a student was supposed to be present in the lesson or no because it only knows the current group_id of a student.

To fix this we need to consult the enrollments table. Enrollments tell us when the student has joined or left particular group.
Student_lessons should only exist for students while they were active in a particular group.

If a student joined the group 7 days ago, they should have student_lessons for all lessons in the last 7 days, but none for lessons that happened before in that group.

This will require some tinkering with SQL. I suspect this will make the query MUCH less performant but the next step will be to materialize it and setup a refresh schedule