Improve comment reply handling

[akirk]

This adds a number of other changes in coordination with https://github.com/Automattic/wordpress-activitypub/pull/766 and https://github.com/akirk/friends/pull/318:

• Numeric ids are automatically ensured for accounts and media attachments,
• This is now also done for multiple statuses (i.e. the mastodon_api_statuses filter),
• created_at on Account and Status will default to now when not specified,
• The rewritten in_reply_to_id is actually used as comment_post_ID,
• adds a test to ensure that a in_reply_to_id arrives as a comment.