erp5_core: use the html_viewer when the field is non editable
Stop using ck_editor in read_only to display HTML. This allows to get rid of the ck_editor iframe, allowing a faster load and removing the iframe scroll. The maximise button is no more needed in such case. It also removes all ck_editor links handling: links can be open in another browser tab now. This changes impacts both xhtml style and erp5js.
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