[FIX] website, website_sale: preconfigure Dynamic Products w/ 1st option

Before this commit the Dynamic Products snippet was displaying a message
box instead of its content until its template and product category were
selected.

After this commit the Dynamic Products snippet is automatically
configured on drop with the first available template and the first
available product category. The message box only appears if no product
category exists yet. Also an "All Products" category was added as first
element of the list to make the snippet usable before categories are
defined.

task-2439295

closes odoo/odoo#65342

X-original-commit: db3ced474e44b5a8188f0d77521bfcaee2575060
Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>