Added setting use_unique_wiki_id and property wiki_id to CourseDescriptor.

Currently the not-always unique wiki_slug setting is used for a course's root slug.
wiki_id returns wiki_slug if use_unique_wiki_id is False and CourseLocator.package_id
otherwise. This way old courses can continue to use the existing slugs and new courses
can opt-into unique slugs.

LMS-2136