Add Danbooru::URL class.

Introduce a Danbooru::URL class for dealing with URLs. This is a wrapper
around Addressable::URI that adds some additional helper methods. Most
significantly, the `parse` method only allows valid http/https URLs, and
it returns nil instead of raising an exception when the URL is invalid.

app/models/post.rb

@@ -314,10 +314,7 @@ class Post < ApplicationRecord
     def source_domain
       return "" unless source =~ %r{\Ahttps?://}i
 
-      url = Addressable::URI.parse(normalized_source)
-      url.domain
-    rescue StandardError
-      ""
+      Danbooru::URL.parse(normalized_source)&.domain.to_s
     end
   end