Refactored permissions check for course_wiki pages.

CourseRole names have a new format (type_org.number.run). Previously
when checking if a user was staff for a course wiki type_org/number/run
and type_number format role names were checked by parsing user group names.

This logic has been refactored to first fetch all courses which use the
particular wiki_slug and then use courseware.access.has_access to check if the
user has staff permissions on any of the courses.

LMS-2136