dmails: replace hard deletions with soft deletions.

Turn deletions into soft deletions (set the is_deleted flag) instead of
hard deletions (remove from database). The is_deleted flag actually
already existed, but it was never used before.

config/routes.rb

@@ -110,7 +110,7 @@ Rails.application.routes.draw do
       put :cancel
     end
   end
-  resources :dmails, :only => [:new, :create, :update, :index, :show, :destroy] do
+  resources :dmails, :only => [:new, :create, :update, :index, :show] do
     collection do
       post :mark_all_as_read
     end