Provide access to the course_id directly on XModules/Descriptors

Fixes [BLD-456] and [LMS-1392], but making sure that we're always asking
XModules about their course_id (since the ModuleSystem has access to
that data).
Merge pull request #1541 from cpennington/hotfix-conditional-course-id-access

Provide access to the course_id directly on XModules/Descriptors