Added get_courses_for_wiki to xml modulestore and mixed modulestore.

get_course_for_wiki(wiki_slug) returns all courses with that wiki_slug value.

LMS-2136
Refactored access check for for course_wiki.

CourseRole names have a new format (type_org.number.run). Previously
when checking if a user was staff for a course wiki only 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 matching the
wiki_slug and then using courseware.access.has_access to check if the
user has staff permissions on any of the courses.

LMS-2136