Improve all results page performance by a new cache table for best ex…

[sayravai]

…ercise points per student.

Description

What?

Currently the All results page may not load at all for large courses, as there will be a server timeout while waiting for the results. The slowness is due to the database operations required to gather the best submissions per each student, so this PR introduces a caching table for the points of best submissions for each student/exercise. These values can be read directly from this cache table, so the All results page loads magnitudes faster. The downside is, that the cache table needs to be written on every submission, which may get slow due to the required index updates. I'm submitting this PR as draft so the performance can be further tested before deciding whether this approach is suitable for our use case.

There is a helper script (exercise/management/commands/backfill_exercise_user_points.py) for populating a cache table from an existing database. This needs to be run before testing.

Fixes #

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