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/routes.rb

@@ -0,0 +1,11 @@
+# Allow Rails URL helpers to be used outside of views.
+# Example: Routes.posts_path(tags: "touhou") => /posts?tags=touhou
+
+class Routes
+  include Singleton
+  include Rails.application.routes.url_helpers
+
+  class << self
+    delegate_missing_to :instance
+  end
+end