course_wiki: Added function to check if a user is course staff for an article
course_wiki: Course staff can manage/delete articles for their courses only; students cannot delete articles.
Some code and documentation cleanup
course_wiki: Only search for groups with name containing course_slug minus underscore if the couse_slug prefix is numerical
course_wiki: Created seperate method to calculate course_wiki_slug
course_wiki: Added tests for staff access
Refactored course_wiki/settings
pylint fixes