sources: factor out Source::URL::Newgrounds.

Also fix it so that the image URL is set as the source for Newgrounds
posts, not the page URL. It's possible to generate the page URL from the
image URL (except for images after the first in multi-image posts).

* Page: https://www.newgrounds.com/art/view/natthelich/weaver
* Image: https://art.ngfiles.com/images/1520000/1520217_natthelich_weaver.jpg?f1606365031

app/logical/source/url.rb

@@ -20,6 +20,7 @@ module Source
     SUBCLASSES = [
       Source::URL::Twitter,
       Source::URL::HentaiFoundry,
+      Source::URL::Newgrounds,
       Source::URL::Plurk,
       Source::URL::Skeb,
       Source::URL::TwitPic,