Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

fix cohorts in create, and read, and update regular expressions to fix courses with periods not working in General commentable

Also fixed up some random bugs with content_info, and changed pagination code a little. Needs comments service update for the subscribed threads API call.