routes: replace hardcoded routes in models with route helpers.

Add a Routes module that gives models access to route helpers outside of
views, and use it to replace various hardcoded routes.

app/logical/upload_service/replacer.rb

@@ -11,7 +11,7 @@ class UploadService
     end
 
     def comment_replacement_message(post, replacement)
-      %("#{replacement.creator.name}":[/users/#{replacement.creator.id}] replaced this post with a new image:\n\n#{replacement_message(post, replacement)})
+      %("#{replacement.creator.name}":[#{Routes.user_path(replacement.creator)}] replaced this post with a new image:\n\n#{replacement_message(post, replacement)})
     end
 
     def replacement_message(post, replacement)