Adds a codepath that would store them as trackback or pingback, depending on the metadata available on the sending page.

Separate the MF2 parsing into its own function. Since the origin url is fetched only there, we can easier make that optional later.

Only covers one case: A webmention with an in-reply-to that contains the author's h-card.

The spamblock plugin will check for it later

Also adds the trackack link-rel, that was so far only in 2k11