…diverse posts

When replying to a remote post (resolved via search), the in_reply_to_id
filter maps the numeric ID back to the original URL. api_submit_comment
was blindly setting comment_post_ID to that URL string and calling
wp_insert_comment() with invalid data, triggering:
  preg_match(): Passing null to parameter #2 ($subject)
The reply was also never federated since no proper post was created.

Fix api_submit_comment to skip comment creation when comment_post_ID
doesn't correspond to a real local post, falling through to api_submit_post.

Fix prepare_post_data to handle a URL-based in_reply_to_id by prepending
an activitypub/reply block to the post content (rather than setting an
invalid post_parent), so the ActivityPub plugin can federate the reply
to the correct remote URL.