Update to the Candidate Recommendation: ActivityPub

The Social Web Working Group invites implementations of an updated Candidate Recommendation of ActivityPub. The ActivityPub protocol is a decentralized social networking protocol based upon the [ActivityStreams] 2.0 data format. It provides a client to server API for creating, updating and deleting content, as well as a federated server to server API for delivering notifications and content. The two normative changes are to improve compatibility with the Mastodon federated social network implementation, in response to feedback from Mastodon developers.