Add webmention support

![](http://dissociatedpress.com/wp-content/uploads/2011/02/oh-the-huge-manatee.jpg)
So here's the trouble. In this method, `webmention` is set by this helper:

```
def webmention
  @webmention ||= Webmention.find(params[:id])
rescue ActiveRecord::RecordNotFound
  halt 404
end
```

If verification runs and fails, the record is deleted from the database (as it should), but `webmention` is never reloaded. So it's _always_ true, and the browser will always redirect to `webmention_path(webmention)`.

I thought about doing `webmention.reload` but that'd require a begin/rescue block on an `ActiveRecord::RecordNotFound`. Felt kinda gross.

What do you think?