Resources for invalid courses causes error

[smartspot2]

After the valid_until date passes for a course, it is no longer given as a part of the /api/courses query (it is excluded from the queryset), but the resources seem to still be accessible. This causes some weird behavior on the resources page, and can occasionally cause errors if no course is valid.

Some options:

• Keep displaying course resources for courses past their valid_until date, but also keep the current behavior of removing them from the course menu, etc.
• Remove course resources entirely from requests past their valid_until date, just like how we exclude courses from the course views.
• Change the behavior of requests for courses, and instead always give all courses present in the database, even if they're past their valid_until date. This would mean that the valid_until date is purely for attendances, and not for display or any requests for data.