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.
In the future, I believe that pasting in the embed code will need to be a supported use case even if we have more user-friendly ways of defining the behaviors... I imagine that either the embed_code is specified (as today) or other fields are provided to allow one to specify different options. In that case, the embed_code will be inferred (built) from the options present.

One could go from field_specs => embed_code easily - once embed_code is provided, then the xblock could be considered in "classic" (like today) mode.

If we want, we could try to parse the embed_code back into fields, but I'm not sure that we need to.