Reinstate "Remove CourseOverview.get_from_id[s]_if_exists (#22918)"

This reverts commit cdb06198,
which itself reverted 3ca00621.

The original commit (3ca006) was reverted because it was suspected
that it was causing unexpectedly-increased memcached usage and
500s in the Gradebook API. It is not clear whether that is actually
the case. We are optimistically reinstating 3ca006 and will monitor
production to see if there is an adverse effect.

MST-105