dmails: allow site owner to read all mails.

Allow site owner to read dmails sent to other users. This is make it
easier to investigate spam without having to drop into the dev console.

test/functional/dmails_controller_test.rb

@@ -99,6 +99,11 @@ class DmailsControllerTest < ActionDispatch::IntegrationTest
         assert_response 403
       end
 
+      should "show dmails to the site owner" do
+        get_auth dmail_path(@dmail), create(:owner_user)
+        assert_response :success
+      end
+
       should "mark dmails as read" do
         assert_equal(false, @dmail.is_read)
         get_auth dmail_path(@dmail), @dmail.owner