v2: Syndication UI

From today's Syndication hack day, we agreed on some basic requirements for v2 UI that need to stay in place. All of these already exist in V1. This came out of a group conversation led by Tatyana G, who runs works on product at CBS.
- Syndication Settings (/wp-admin/options-general.php?page=push-syndicate-settings): The master site needs to be able to set what Post Types are syndicated, and a tick box to delete all the pushed posts when the master post is deleted.
- Edit Post Page: Has a metabox to select which groups of sites the post will get pushed to. Display top 5, and then click to show more. Option to check/uncheck all.
- Add Site: oAuth2 authentication for WP API, and which Site Group a site gets added to

These items do not exist in v1, but are needed for V2:
- All Posts Page: For each post, show which group of sites a post was syndicated to.
- Ability to set up email/Slack notifications for posts when they are syndicated