sources: refactor normalize_for_source.

`normalize_for_source` was used to convert image URLs to page URLs when displaying sources
on the post show page. Move all the code for converting image URLs to page URLs from
`Sources::Strategies#normalize_for_source` to `Source::URL#page_url`.

Before we had to be very careful in source strategies not to make any network calls in
`normalize_for_source`, since it was used in the view for the post show page. Now all the
code for generating page URLs is isolated in Source::URL, which makes source strategies
simpler. It also makes it easier to check if a source is an image URL or page URL, and if
the image URL is convertible to a page URL, which will make autotagging bad_link or
bad_source feasible.

Finally, this fixes it to generate better page URLs in a handful of cases:

* https://www.artstation.com/artwork/qPVGP instead of https://anubis1982918.artstation.com/projects/qPVGP
* https://yande.re/post/show?md5=b4b1d11facd1700544554e4805d47bb6s instead of https://yande.re/post?tags=md5:b4b1d11facd1700544554e4805d47bb6
* http://gallery.minitokyo.net/view/365677 instead of http://gallery.minitokyo.net/download/365677
* https://valkyriecrusade.fandom.com/wiki/File:Crimson_Hatsune_H.png instead of https://valkyriecrusade.wikia.com/wiki/File:Crimson_Hatsune_H.png
* https://rule34.paheal.net/post/view/852405 instead of https://rule34.paheal.net/post/list/md5:854806addcd3b1246424e7cea49afe31/1

app/models/post.rb

@@ -307,15 +307,16 @@ class Post < ApplicationRecord
       end
     end
 
+    def parsed_source
+      Source::URL.parse(source) if web_source?
+    end
+
     def normalized_source
-      return source unless web_source?
-      Sources::Strategies.normalize_source(source)
+      parsed_source&.page_url || source
     end
 
     def source_domain
-      return "" unless web_source?
-
-      Danbooru::URL.parse(normalized_source)&.domain.to_s
+      parsed_source&.domain.to_s
     end
   end