Added support for publishing newsfeed posts via the ActivityPub spec.
Implements a basic ActivityPub support for newsfeeds posts.
Based upon the W3C ActivityPub spec: https://w3c.github.io/activitypub/#create-activity-outbox
NOTE: It was very hard for me to find actual ActivityPub services to actually test this against, so I installed Hubzilla's Docker image and tested it locally. And ... it seems to work!
What's still needed are Read routes via new Minds.com's routes, but I figured that was beyond the scope of this PR.
I don't think that I did DI per the project's standards. I would appreciate guidance.