[FIX] web_editor: append nbsp in text node

In JavaScript, createTextNode() escapes it's contents, so instead of
appending ' ' as text, we should append the character corresponding
to a non-breaking space. Alternative would be to use innerHTML.

odoo-helpdesk ticket 3372476

closes odoo/odoo#125089

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>