[FIX] web_editor: hide codeview when readonly

In 2389f77c
containsComplexHTML was added to allow editing of html content
that would otherwise break when inserted as-is in a DOM

For this we added a rule to always display the code view toggle button.
This should not be the case when the field is marked as readonly.

Switching to the code view in readonly mode will cause a traceback.

task-3255777

Part-of: odoo/odoo#118423