Allow selecting courses on event exports.

Reads the `courses` specifier on the configuration file, and only includes the listed courses in the export.
What subset of the org events are you trying to include in the per-course events?  It's clearly going to be a subset, since you don't include all the other cases.  And it can't include any that didn't already find the org, because you've already skipped them.  (And we can't change the org code because it "must" be idempotent.)

But in terms of "old" data, why only try for server events that appear as "/courses/<course_id>"?  First of all, I would expect a bunch would have more context following the course_id, but this includes the rest of the url.   It also assumes that it starts with /courses/, which is a subset of events.  And even if you got that (by using [2:5] or something similar), why just look at a subset of old server events and not also get old browser events? 

Why not just use:  course_key = opaque_key_util.get_course_key_from_url(url)  where url = event_type if server and url = page if browser.