[FIX] point_of_sale: correctly assign and display html for the customer display

Commit f4200f37 aimed at avoiding 414 errors on too long uri.
But it brought up something else: on a new pos.config, the customer display html was not set due to that commit meddeling with the iframe's url.

This present commit forces the set up of the html field on create and write (according to the values, as usual)
the price to pay is to not being able to edit the html right away