edx/edx-platform — pull request #1980 by dmitchell (2013-12-17 17:17:14+00:00)
Title: Rename BlockUsageLocator's block_id field from usage_id

usage_id implies self-sufficiency. The block_id is relative to its structure.

@cahrens @singingwolfboy please review. Verifying completeness is a pain, but thankfully we have almost 100% code coverage for all of the BlockUsageLocator code.