DO NOT MERGE (FOR INITIAL REVIEW ONLY): Add ability to cap number of enrollments in a course

@ormsbee @dianakhuang mind doing a quick glance over this? This feature is to allow courseware authors to specify a cap on the total number of active enrollments in a course. Some of our B2B folks have expressed concerned that our contracts specify a "# seat licenses" (with overruns being invoiced out) but the application doesn't cap enrollments or otherwise provide a means to limit that liability.

I have to write tests before the full PR, but I wanted to get some quick feedback on the feature/approach.

Thanks,

Chris