Student dashboard views.

Generate the student dashboard using PostgreSQL views. Primarily a view called membership_calculation that gathers all the summaries that are specific to the given student and course of the current dashboard.

These summaries are then used in the `current_student_data` where individual queries for each summary would have been run.

Also created `membership_scores` that contains scores grouped by assignment type, a query used by the predictor. The generation of the JSON needed for the predictor has been moved from `User` to `StudentData`.

Also created a `released_grades` view to simplify some of the other queries. It filters grades using the released or graded and no release necessary condition that is found throughout the code. I'm not using that view enough in the SQL, there are more opportunities to use it.