Add proposal translations

Adapt retire form to include needed translations and move validations
from controller to model.

Also change sanitizable concern to sanitize not marked for destruction
translations.

spec/models/proposal_notification_spec.rb

@@ -146,7 +146,9 @@ describe ProposalNotification do
       it "returns false if the resource is retired" do
         notification = create(:notification, notifiable: notifiable)
 
-        notifiable.proposal.update(retired_at: Time.current)
+        notifiable.proposal.update(retired_at: Time.current,
+          retired_explanation: "Unfeasible reason explanation",
+          retired_reason: "unfeasible")
         expect(notification.check_availability(proposal)).to be(false)
       end