Create attachment model and hook it up to todo

An attachment has write permissions on group so that managing
attachments work from different users in same group, i.e. user apache
and user mail.

test/models/attachment_test.rb

@@ -0,0 +1,7 @@
+require 'test_helper'
+
+class AttachmentTest < ActiveSupport::TestCase
+  # test "the truth" do
+  #   assert true
+  # end
+end