Add proposals translation interface

* Convert proposal form into translatable one.
 * Adapt translatable shared spec to define an owner when running at
   frontend feature specs.
 * Remove old attributes from strong parameters.

spec/features/proposals_spec.rb

@@ -15,6 +15,11 @@ describe "Proposals" do
   context "Concerns" do
     it_behaves_like "notifiable in-app", Proposal
     it_behaves_like "relationable", Proposal
+    it_behaves_like "translatable",
+                    "proposal",
+                    "edit_proposal_path",
+                    %w[title summary],
+                    { "description" => :ckeditor }
   end
 
   context "Index" do