Opaque-keys v2: package, deprecate smartly

@cpennington my attempt to move all of the opaque key stuff into one installable module, register SSCK and Location as the fallback key class, and have the key serialize as deprecated if it was deserialized from deprecated.

I'm thinking of putting this in as the first PR before combining the Locator ones w/ the Location ones.

I need to add a few quick tests of CourseKey ala the UsageKey test. I wanted to get your early review. Invite others if appropriate.

STUD-1625