block.get_parent() and group access enforcement

@cpennington @dmitchell @andy-armstrong   I'm rebuilding the commits on my branch for ease of review.  The first implements the plumbing necessary for get_parent() to work.

Questions:
* smuggling `parent_url` inside the `cached_metadata` seems to work - but only some of the time.  Therefore an (extremely naive) just-in-time lookup is added as a fallback.  Sadly it seems `load_item` executes multiple times for the same block during the same modulestore lifecycle, therefore we have multiple objects in memory for the same block, but different identities (as evidenced by the need in tests to compare equality by blocks' locations instead of the blocks themselves).  Is this by design?  If not, is there any realistic way to address it without a major effort?  I don't yet feel comfortable enough making changes to this code to guess.
* I suspect that I should be testing get_parent with each modulestore implementation, is that right?
* Are the tests for get_parent (for any given modulestore) exhaustive enough?
In particular, 
1. create a course w/ >= 2 verticals and >= 1 subelement
2. move the subelement from one vertical to the other but don't publish either vertical after the move
3. load the subelement via the draft store (branch draft_preferred) and verify that get_parent returns the new parent
4. load the subelement via the published store and verify that get_parent returns the old parent