feat: add explicit courserun_key parameter to /event endpoint

We add 'courserun_key' (aka "course_id" though that's technically a
misnomer) as an optional parameter to the /event endpoint url. If it
is not present, it will still be parsed out of the url, if the url is
of the right format.

Additionally, Logger.log() in js adds this parameter to its /event
call, pulling it from the $$course_id global.

This provides opportunity for MFEs to (separately) provide the key
without concern about url parsing.

TNL-7752