Receiving Salmentions

[dshanske]

This Replaces https://github.com/pfefferle/wordpress-semantic-linkbacks/issues/40

Related to #281 and #198, in order to support things like salmentions, we need to continue to store, not necessarily the entire source of a source URL. We can store comments of a received webmention as threaded comments.

This means, now that we are parsing mf2, we need to extend it to parse children and generate threaded comments. It means on updating a comment object, we must also now update its children.