Implementation of old-style keys as OpaqueKeys (Location and course_id)
Change Locators into UsageKeys and CourseKeys
Create json encoder class to handle OpaqueKeys
with special handling of Locations and SlashSpecifiedCourseKeys
Fixed CourseKeyField validation to validate CourseKeys properly
Make LocationKeyFields validate properly
Fix all uses of UsageKeys and CourseKeys, and all of their tests.
Make Studio support opaque-keys