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.
Why not getting it for the user via the API? https://developers.google.com/drive/v2/reference/files seem to have `embedLink` for this, and it might make it easier to expand the capabilities of this XBlock going forward - it we store the id of the document we can query the API for other things related to this doc in the future, while if we store copy & pasted HTML code it might be harder to do.