Support updated and deleted incoming webmentions

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Right now the primitive validations performed by the webmention function are actually too strict. When a page is mentioning page is deleted, the webmention spec requires that the author resend the webmention so that the receiver will re-run any checks and determine that the mention is gone.

There are some options...

The simplest is to create an issue even when there's no mentioning page, and include a field to show that the source page is gone and so that I know to remove a mention.

Pros:

• Easy to implement. Probably just a few lines changed.
  Cons:
• Susceptible to spam.

The next, slightly more complex, option is to check the target page for an existing mention. When the mention already exists and it is found in the source page too, the mention might be for an update. When this happens it could either be ignored, or used as an opportunity to check if data used in the target page for the mention is up to date. When the mention is in the target page but the source page is gone, this constitutes a deletion.

The even more complex option is to do the above, but also check for any open webmention issues. When an issue exists for a particular source and target, it can be replaced (in the case of an update) or removed (if the mention is deleted). This handles the case when a mention is queued as an issue for me to look at, but updates and/or deletions happen before I can look at it.