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/twitter.rb

@@ -26,7 +26,7 @@ class Source::URL::Twitter < Source::URL
   # https://developer.twitter.com/en/docs/developer-utilities/configuration/api-reference/get-help-configuration
   RESERVED_USERNAMES = %w[home i intent search]
 
-  attr_reader :status_id, :twitter_username, :user_id
+  attr_reader :status_id, :username, :user_id
 
   def self.match?(url)
     return false if Source::URL::TwitPic.match?(url) # TwitPic uses https://o.twimg.com/ URLs
@@ -45,12 +45,12 @@ class Source::URL::Twitter < Source::URL
     # https://twitter.com/Kekeflipnote/status/1496555599718498319/video/1
     # https://twitter.com/sato_1_11/status/1496489742791475201/photo/2
     in "twitter.com", username, "status", status_id, *rest
-      @twitter_username = username
+      @username = username
       @status_id = status_id
 
     # https://twitter.com/motty08111213
     in "twitter.com", username, *rest
-      @twitter_username = username unless username.in?(RESERVED_USERNAMES)
+      @username = username unless username.in?(RESERVED_USERNAMES)
 
     # https://twitter.com/intent/user?user_id=1485229827984531457
     in "twitter.com", "intent", "user" if params[:user_id].present?
@@ -58,7 +58,7 @@ class Source::URL::Twitter < Source::URL
 
     # https://twitter.com/intent/user?screen_name=ryuudog_NFT
     in "twitter.com", "intent", "user" if params[:screen_name].present?
-      @twitter_username = params[:screen_name]
+      @username = params[:screen_name]
 
     # https://twitter.com/i/user/889592953
     in "twitter.com", "i", "user", user_id
@@ -101,9 +101,17 @@ class Source::URL::Twitter < Source::URL
     "#{site}/#{@file_path}:orig"
   end
 
+  def page_url
+    if username.present? && status_id.present?
+      "https://twitter.com/#{username}/status/#{status_id}"
+    elsif status_id.present?
+      "https://twitter.com/i/web/status/#{status_id}"
+    end
+  end
+
   def profile_url
-    if twitter_username.present?
-      "https://twitter.com/#{twitter_username}"
+    if username.present?
+      "https://twitter.com/#{username}"
     elsif user_id.present?
       # "https://twitter.com/i/user/#{user_id}
       "https://twitter.com/intent/user?user_id=#{user_id}"