Re-enable "Diff Object" "Diff Module Objects" for users

This is used and should not be disabled.

There was an error when clicking this dialog, fix that error instead.

See merge request !1301
10 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

01:35:42

passed ERP5.PerformanceTest-Master

00:23:37

failed ERP5.UnitTest-Master

02:49:29

failed ERP5.UnitTest-Master.Medusa

02:56:01

passed SlapOS.Eggs.UnitTest-Master.Python2

00:16:19

passed SlapOS.Eggs.UnitTest-Master.Python3

00:08:03

passed SlapOS.Eggs.UnitTest-Master.Python2

00:25:07

passed SlapOS.Eggs.UnitTest-Master.Python2

00:23:53

passed SlapOS.Eggs.UnitTest-Master.Python3

00:09:40

passed SlapOS.Eggs.UnitTest-Master.Python3

00:16:58