Introduce a Proposable concern

The Proposable concern defines a relation to a Proposition and defines
scopes that translate to the statuses of the Proposition.

app/models/concerns/proposable.rb

@@ -0,0 +1,11 @@
+module Proposable
+  extend ActiveSupport::Concern
+
+  included do
+    has_one :proposition, as: :proposable
+
+    Proposition.defined_enums["status"].keys.each do |status|
+      scope status.to_sym, -> { joins(:proposition).where(propositions: {status: Proposition.defined_enums["status"][status]}) }
+    end
+  end
+end