website: set content editable false in selection field

<b>Description:</b> In website module set <code>contenteditable = false</code> in selection field.

<b>Task:</b> https://www.odoo.com/web#id=9015&view_type=form&model=project.task&menu_id=3942&action=327 
<b>Pad:</b> https://pad.odoo.com/p/r.MiPr0wuj3FtjI5EL