Refactor source normalization

* Move the source normalization logic out of the post model
  and into individual sources' strategies.
* Rewrite normalization tests to be handled into each source's test,
  and expand them significantly. Previously we were only testing
  a very small subset of domains and variants.
* Fix up normalization for several sites.
* Normalize fav.me urls into normal deviantart urls.

app/logical/sources/strategies/pixiv.rb

@@ -213,6 +213,12 @@ module Sources
         illust_id.present? || novel_id.present? || fanbox_id.present? || fanbox_account_id.present?
       end
 
+      def normalize_for_source
+        return if illust_id.blank?
+
+        "https://www.pixiv.net/artworks/#{illust_id}"
+      end
+
       def tag_name
         moniker
       end