webmentions for updated posts are out of sync

[aaronpk]

when a checkin is updated, such as when adding a photo, the swarm comments change, e.g. how many coins are awarded for a photo. The current way this works means sometimes duplicate webmentions will be sent to the post after the checkin is updated, and the webmentions have different source URLs. This can be fixed one of two ways:

• update the comment text and send a webmention update
• delete the previous webmentions so the new ones don't look like duplicates