sources: add image_url?, page_url?, and profile_url? methods.

Add methods to Source::URL for determining whether a URL is an image
URL, a page URL, or a profile URL.

Also add more source URL tests and fix various URL parsing bugs.

app/logical/source/url/art_station.rb

@@ -65,7 +65,7 @@ class Source::URL::ArtStation < Source::URL
   end
 
   def image_url?
-    subdomain.starts_with?("cdn")
+    subdomain.to_s.starts_with?("cdn")
   end
 
   def full_image_url(size = "original")