Use the ActivityPub blog user as an actor if set to blog profile only

[akirk]

Fixes #410

Instead of $actor = \get_author_posts_url( $user_id ); we now use $actor = \Activitypub\Collection\Actors::get_by_id( $user_id ); which also work for the ActivityPub blog actor.

[akirk]

@pfefferle is this a good way to do it?

[akirk]

@pfefferle my question was more whether it's ok to use such an Activitypub\Collection\Actor for $activity->set_actor( $actor );