This is a pretty simple way for us to track which webmentions are sent
by Ghost. Although it's easily spoofed, so are other approaches like
using a header (e.g. User-Agent). If we find that this data is being
spoofed we can look at different approach.
Becuase our receiving implementation stores the payload of the
Webmention, we'll be able to know inside Ghost which Mentions
originated from another Ghost installation, which is useful for stats
and gives us the possibility to display that information in the feed.
Longer term we might want to consider storing this data in a separate
column for Mentions, rather than the `payload` column - but that is
outside the scope of this change.
