[FIX] point_of_sale: hide QWeb header/footer from non-ESC/POS receipt

We allow users to specify QWeb in the header and footer fields of the
receipt. Because the browser receipt and the ESC/POS receipt use a
different format (HTML and ESC/POS XML respectively) we don't render
this QWeb to both receipts. Currently it is only rendered for the
ESC/POS receipt. When this is the case however, we shouldn't display the
unrendered QWeb on the regular browser receipt. This commit simply hides
it.

Perhaps in the future we can provide an additional parameter in the
<DOCTYPE declaration that allows the user to specify whether he wants
the QWeb to apply to the HTML receipt and/or the ESC/POS receipt.

opw-678170