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?
This is not quite right (see above comments). The trouble is the caller of this is being too smart and setting revision to `published` if the item is one of the DIRECT_ONLY categories; so, we can't believe the caller as to its intent unless we ensure that all DIRECT_ONLY ones get written to `published`. If the branch is set to `draft`