Introduce a PropositionAccepting concern

It's sole purpose is to denote models that can be associated with a
proposable model.

app/models/concerns/proposition_accepting.rb

@@ -0,0 +1,7 @@
+module PropositionAccepting
+  extend ActiveSupport::Concern
+
+  included do
+    has_many :propositions, as: :proposition_accepting
+  end
+end