Pre-load the course overviews attached to CourseEnrollments on the
Student Dashboard, if possible. This will only grab the CourseOverviews
that already exist, and will not generate new ones. Any missing
CourseOverviews fall back to the lazily-created one-at-a-time behavior
they've always had. That's mostly because I wanted to optimize for the
common case in the least invasive way possible, and I don't want to get
caught up in locking issues.
