Allow selecting courses on event exports.

Reads the `courses` specifier on the configuration file, and only includes the listed courses in the export.
AH, I was referring to old events. The courses requested were from 2013, which probably does not have the context. The way I wrote the code it will check first if the field is there, and if not then check the URL.

My thought was that since we are not going to give them all the events (because it is too much work to write a detailed `get_course_id`, at least we give them a large number of implicit ones.