Modify course properties to make logic clearer

[smartspot2]

Currently, there are section_start, valid_until, enrollment_start, and enrollment_end fields on the Course object.

The enrollment_start and enrollment_end fields behave like they should, along with section_start. However, the valid_until field seems to be used for a bunch of different purposes, without much clarity on how it's actually being used. There should probably be a section_end field, denoting when attendances should stop being created (since section_start denotes when attendances start being created). If we want to control visibility, then that should probably be the desired purpose of valid_until.

This could be merged/incorporated into the archiving project (#244, #270), since the valid_until field could probably be used to denote when a semester "ends" for a course.

There could also be several other ways to improve the semantics of the Course object fields not mentioned here, and I'd be open for discussion on it as well.