Proper activity audience targeting

At the moment, the recipients for an activity are always actor.followers, and to and cc are hardcoded in _activity.activitypub.jbuilder. We should implement proper addressing, so that we can do public posts, direct, follower-only, etc.

References:

• https://www.w3.org/TR/activitypub/#public-addressing
• https://www.w3.org/TR/activitystreams-vocabulary/#audienceTargeting
• https://docs.joinmastodon.org/spec/activitypub/#Mention

I'd also really like a locally-public-only option, but I don't think that's in the ActivityPub spec, so it might need to be a custom extension.