Added get_courses_for_wiki method.

LMS-2136
Added get_courses_for_wiki(self, wiki_slug) to mongo, xml and mixed modulestores.

This method returns a list of courses which use the particular wiki_slug. It is used for
checking permissions in the course_wiki app.

LMS-2136
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
When creating a course in studio set a unique wiki_slug for it.

Currently wiki_slug is set to course number. However, since multiple
courses can have the same number this may lead to clashes. So wiki_slug
will be set to org.course.name. To maintain the active wiki_slugs for
xml courses this cannot be changed in the CourseDescriptor.

LMS-2136
Export CourseDescriptor wiki_slug field to xml.

LMS-2136
Merge pull request #2559 from edx/usman/lms2136-wiki-access

Refactor of permissions checking for wiki pages.