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
@dmitchell We are using [django-wiki] (https://github.com/edx/django-wiki) here and its views are not written in edx-platform, ```course_id``` or ```course_key_string ``` passes to wiki fns as a keyword argument and these wiki fns are not accepting an argument like ```course_key_string``` or ```course_id```.