Allow selecting courses on event exports.

Reads the `courses` specifier on the configuration file, and only includes the listed courses in the export.
The get_org_id logic below is pretty hairy and not alway correct, but was left in place because it is what we used in the past (and we didn't want to select different events than we did before).  But this method is not going to pull out the same course_id's that the get_org_id implicitly does.  This version is only going to pull a subset of course_ids out.  And in particular it won't do the right thing for any new-style id's.