Add a Google Comments source

[nokome]

In #818 we propose adding the Review model. Our intention, as with Files, is that users can specify external Sources for Reviews. One source for a Review is one or more comments on a Google Doc (or other Google Drive app).

Comments are associated with a file (i.e. GET https://www.googleapis.com/drive/v2/files/fileId/comments) and we would normally want to only include comments from a subset of users (so we could separate multiple comments on a doc into separate reviews). So, the GoogleCommentsSource would need to have the following fields:

• google_id (inherited from GoogleDriveSource ?)
• reviewers (a list of emails, names, or regexes that can be used to filter comments)

I propose that we treat the first comment by a reviewer as the content of the review, assume that it is Markdown formatted, and either:

• decode the Markdown to a Node and link that as the main content of the Review instance
• write the Markdown to a File and link that as the main content of the Review instance

The related Google API docs are:

• https://developers.google.com/drive/api/v2/reference/comments
• https://developers.google.com/drive/api/v2/reference/replies

Requires #818.

[alex-ketch]

For simplicity in the initial version, we the first comment in the document will be used as the review body/summary. Not the first chronological comment as I had initially interpreted 😅
Also, it seems there isn't a need for custom CommentsSource models. Instead the comments are simply fetched/stored in the database by regular worker jobs.

[nokome]

Closing this as initial version has been completed as part of #868.