Improve read only HTML

We receive feedback from users saying that "when I view a mail message in event module, I see ugly html source", this was because user had set preferred text editor to monaco. Improve a bit monaco integration so that we display something better than code in a `<pre>`  for non editable editors.

We had another feedback that when using CKEditor as preferred text editor in old UI, users can not click on links when viewing an html Mail Message. But it works in new UI. I did something minimal so that this looks working until the migration to new UI is complete.

/reviewed-on nexedi/erp5!1011