This makes get_my_courses() faster/lighter in 2 ways
- We fetch a ton less data from the DB
- We store a tone less data in the Session
In a test environment, with a student enrolled in 3 _empty_ courses
(just created, empty modinfo, etc), this shaves 50% off the session
size on-disk.
The problem is callers that may be expecting a given field to appear by
default. Next step: walk all callers...
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