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.rb

@@ -24,5 +24,9 @@ module Sources
     def self.canonical(url, referer)
       find(url, referer).canonical_url
     end
+
+    def self.normalize_source(url)
+      find(url).normalize_for_source || url
+    end
   end
 end