Minor tweaks to WebmentionHandler.
Minor tweaks to WebmentionHandler.

* Do basic sanity checks *before* fetching the source document.
* If source is a valid post, also check for its referenced_url to be equal to target
hmans/pants,KlausTrainer/pants,hmans/pants,hmans/pants,spacetraveler/pants,KlausTrainer/pants,spacetraveler/pants,spacetraveler/pants,KlausTrainer/pants