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/skeb.rb

@@ -42,6 +42,10 @@ class Source::URL::Skeb < Source::URL
     end
   end
 
+  def profile_url
+    "https://skeb.jp/@#{username}" if username.present?
+  end
+
   def image_url?
     domain.in?(%[imgix.net amazonaws.com])
   end