ActivityPub API: Interaction with remote activities

[mediaformat]

What

Interact with remote posts via ActivityPub API:

• Announce
• Like
• Reply
• Vote

Related #3466

Why

In order to participate with the broader Fediverse, outside of our own posts

How

There seem to be 2 main blockers for interacting with remote posts.

1. The $result of \apply_filters( 'activitypub_outbox_' will be a WP_Comment with a comment_post_ID of 0
   

   wordpress-activitypub/includes/collection/class-interactions.php

   Lines 48 to 53 in 45f3472

   	$parent_comment_id = 0;
   	if ( ! empty( $activity['object']['inReplyTo'] ) ) {
   	// Regular reply.
   	$target_url = object_to_uri( $activity['object']['inReplyTo'] );
   	$parent_comment_id = url_to_commentid( $target_url );

   
   $target_url is not guaranteed to be another comment, so additional checks should search for an ap_post. In the case of an activity fetched via proxy, an ap_post will not be found since it exists only as a transient, so a pre comment hook would be needed to save it as an ap_post.
2. Outbox::get_by_object_id() will return null for a WP_Comment
   

   wordpress-activitypub/includes/collection/class-outbox.php

   Lines 283 to 284 in 45f3472

   	public static function get_by_object_id( $object_id, $activity_type ) {
   	$outbox_items = \get_posts(