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/logical/source/url.rb

@@ -16,7 +16,7 @@
 #   url = Source::URL.parse("https://twitter.com/yasunavert/status/1496123903290314755")
 #   url.site_name        # => "Twitter"
 #   url.status_id        # => "1496123903290314755"
-#   url.twitter_username # => "yasunavert"
+#   url.username         # => "yasunavert"
 #
 # @see Danbooru::URL
 module Source
@@ -53,7 +53,7 @@ module Source
     # @return [Source::URL]
     def self.parse!(url)
       url = Danbooru::URL.new(url)
-      subclass = SUBCLASSES.find { |c| c.match?(url) } || Source::URL
+      subclass = SUBCLASSES.find { |c| c.match?(url) } || Source::URL::Null
       subclass.new(url)
     end
 
@@ -78,39 +78,30 @@ module Source
     #
     # @return [String]
     def site_name
-      # XXX should go in dedicated subclasses.
-      case host
-      when /ask\.fm\z/i
-        "Ask.fm"
-      when /bcy\.net\z/i
-        "BCY"
-      when /booth\.pm\z/i
-        "Booth.pm"
-      when /circle\.ms\z/i
-        "Circle.ms"
-      when /dlsite\.(com|net)\z/i
-        "DLSite"
-      when /doujinshi\.mugimugi\.org\z/i
-        "Doujinshi.org"
-      when /fc2\.com\z/i
-        "FC2"
-      when /ko-fi\.com\z/i
-        "Ko-fi"
-      when /mixi\.jp\z/i
-        "Mixi.jp"
-      when /piapro\.jp\z/i
-        "Piapro.jp"
-      when /sakura\.ne\.jp\z/i
-        "Sakura.ne.jp"
-      else
-        if self.class == Source::URL
-          # "www.melonbooks.co.jp" => "Melonbooks"
-          parsed_domain.sld.titleize
-        else
-          # "Source::URL::NicoSeiga" => "Nico Seiga"
-          self.class.name.demodulize.titleize
-        end
-      end
+      # "Source::URL::NicoSeiga" => "Nico Seiga"
+      self.class.name.demodulize.titleize
+    end
+
+    # Convert an image URL to the URL of the page containing the image, or
+    # return nil if it's not possible to convert the current URL to a page URL.
+    #
+    # When viewing a post, the source will be shown as the page URL if it's
+    # possible to convert the source from an image URL to a page URL.
+    #
+    # Examples:
+    #
+    # * https://i.pximg.net/img-original/img/2014/10/03/18/10/20/46324488_p0.png
+    #   => https://www.pixiv.net/artworks/46324488
+    #
+    # * https://images-wixmp-ed30a86b8c4ca887773594c2.wixmp.com/intermediary/f/8b472d70-a0d6-41b5-9a66-c35687090acc/d23jbr4-8a06af02-70cb-46da-8a96-42a6ba73cdb4.jpg/v1/fill/w_786,h_1017,q_70,strp/silverhawks_quicksilver_by_edsfox_d23jbr4-pre.jpg
+    #   => https://www.deviantart.com/edsfox/art/Silverhawks-Quicksilver-126872896
+    #
+    # * https://pbs.twimg.com/media/EBGbJe_U8AA4Ekb.jpg:orig
+    #   => nil
+    #
+    # @return [String, nil]
+    def page_url
+      nil
     end
 
     # Convert the current URL into a profile URL, or return nil if it's not
@@ -134,6 +125,14 @@ module Source
       nil
     end
 
+    def self.page_url(url)
+      Source::URL.parse(url)&.page_url
+    end
+
+    def self.profile_url(url)
+      Source::URL.parse(url)&.profile_url
+    end
+
     protected def initialize(...)
       super(...)
       parse