[FIX] website_sale_options: prevent concurent t-if on same element

  - Install website_sale_coupon
  - Install website_sale_options (in that order)
  - Create a promotion code
  - Add item to cart and go to checkout
  - Add promotion code
  - The quantity for the promotion code is now duplicate and one of them
    can be changed (which is not supposed to be)

Website_sale_options and website_sale_coupon both xpath and t-if the same
element.
The module that has been installed last will have the upper hand on the
condition and only its condition will be applied.

Now, it correctly take care of both conditions.

addons/website_sale_options/views/website_sale_options_templates.xml

@@ -11,7 +11,7 @@
 
 <template id="optional_products" inherit_id="website_sale.cart_lines" name="Optional products">
   <xpath expr="//table[@id='cart_products']/tbody//td[hasclass('td-qty')]/div" position="attributes">
-    <attribute name="t-if">not line.linked_line_id</attribute>
+    <attribute name="t-if" add="(not line.linked_line_id)" separator=" and "></attribute>
   </xpath>
   <xpath expr="//table[@id='cart_products']/tbody//tr" position="attributes">
     <attribute name="t-att-class">'optional_product info' if line.linked_line_id else None</attribute>