Google Drive XBlock - Fake PR for review purposes, do not merge!

@martynjames Created this PR for the review. 

Btw for the future, it's easier to create this when the first commit is empty. Also, if we want to review a series of PRs at once instead of individual PRs, it would be easier to merge the individual PRs in a branch which isn't master, and then create a PR from that branch into master when it's ready -- this way we don't need "fake" PRs at all, they represent the actual review workflow.
I thought simplicity was the reason yep -- my question then is about migration: how do you ensure that users of this approach will be able to migrate to a file_id centric approach without asking them to manually edit the existing XBlocks they would have already added to courses?