Student index page / TestStudentIndexView performance

[richardebeling]

TestStudentIndexView.test_num_queries_is_constant already is pretty slow (2s on my machine) and becomes even slower when increasing the constant 100 evaluations to 1000.

Interrupting the computation, the traceback indicated that the code was stuck in retrieving the course_id_evaluation_weight_sum_pairs in get_evaluations_with_course_result_attributes. This should be a pretty simple and fast query. Maybe the Course subquery isn't optimized out properly by postgres.

In general, the student index page shouldn't load longer than the results index page, but it currently does (1.4s for me vs 0.6s on results)