Rename the repo to xblocks-core? #149
Description
Note: This isn't something we should do now. If we do it, let's do it after the block extractions are complete so that it doesn't disrupt the project.
Background: xblocks-core was one of the original name suggestions for the repo... I think aximprovements had suggested it. I had been against the name, because I thought that the blocks here (HTML, Problem) were only a subset of what we called the "Core" in OEP-57. But since we're now looking to merge in blocks like PDFBLock and DragAndDropV2, "core" would actually be an appropriate name, and it's easier to understand than "contrib" (which was inspired by django.contrib).
I'm not sure what to do about the module, for example xblocks_core.problem and xblocks_core.html. Maybe they should just become top-level, like xblock_problem and xblock_html?