Fixed wiki merge url regex.

TNL-413

By using this ```settings.COURSE_ID_PATTERN``` in wiki url, it passes ```course_id``` as keyword argument to wiki ```merge()```.

Ticket: https://openedx.atlassian.net/browse/TNL-413
Why complicate this v just adding a `course_key_string` arg to the wiki fns? It's bad enough there are 2 existing patterns. I won't hold up the PR but I cringe at the special casing.