models: add deletable concern.

app/logical/concerns/deletable.rb

@@ -0,0 +1,11 @@
+module Deletable
+  extend ActiveSupport::Concern
+
+  class_methods do
+    def deletable
+      scope :active, -> { where(is_deleted: false) }
+      scope :deleted, -> { where(is_deleted: true) }
+      scope :undeleted, -> { where(is_deleted: false) }
+    end
+  end
+end