Source::URL: add profile_url method.

Add a method for converting a source URL into a profile URL. This will
be used for normalizing profile URLs in artist entries.

Also add the ability to parse a few more profile URL formats.

app/logical/source/url/deviant_art.rb

@@ -142,6 +142,10 @@ module Source
       end
     end
 
+    def profile_url
+      "https://www.deviantart.com/#{username}" if username.present?
+    end
+
     def pretty_title
       title.titleize.strip.squeeze(" ").tr(" ", "-") if title.present?
     end