Support staff locking on unit page

STUD-1873
@cahrens: @nasthagiri and I had an interesting discussion about this last week. I admit to not fully understanding all the implications, but it seems to me that Studio should never deal with usage keys except as external inputs, e.g. from URLs. We should only be passing around either xblocks or locations that have the course run so that we can't accidentally use the underspecified version. I agree that at the very least we need to fix the two places you mention, but I have no idea how we can enforce that all other usages are kosher.

In this case, I think we should change ```_save_item``` to take an xblock rather than a usage key. There only seems to be one caller, and it makes the contract much cleaner.

Does that seem reasonable to you both?