Make it possible to select proposals without JavaScript

This way, when JavaScript hasn't loaded (for whatever reason),
administrators can still use this functionality.

spec/factories/legislations.rb

@@ -177,5 +177,9 @@ FactoryBot.define do
     trait :hidden do
       hidden_at { Time.current }
     end
+
+    trait :selected do
+      selected { true }
+    end
   end
 end