Expose Notes via API so that bots can comment or respond to comments#2644

One of the main reasons we’re using Publer is so that our internal bot can post draft content for humans to review and approve. The bot wants to add comments to the post; right now it has to do that in the text of the post, and there’s a danger that it will somehow slip through human editing and get posted with the bot’s comments. If the bot could add comments (notes) to a post, it could add the additional data that the human needs to see (but shouldn’t actually be part of the post) itself; and our humans could also @mention the bot to tell it when it did a good or bad job with a draft to help it improve.