[FIX] website_sale: add index to website_sequence

On a DB with 2M+ products, the computation of the
`_default_website_sequence` [1] takes several seconds. In case of a
product creation in batch, it can negatively affect the whole
server performance.

Adding an index solves the problem.

[1] https://github.com/odoo/odoo/blob/1b9c7fecc5b4031c5d7b9bbe4f26e1e510063d12/addons/website_sale/models/product_template.py#L368



closes odoo/odoo#135030

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>