[IMP] account: display access button in notification emails for users and customers

If applied, this PR will enable button_access for all users and portal_customers, letting them
access the documents directly. The button "View Journal Entries" will appear on the notification
email, and by clicking this the user will be directed to the portal page of the invoice/refund.
All users and portal_customers subscribed to the thread should be able to see this button.

A following refactor by TDE (#82167) has addressed this problem in master (for 15.2).
For stable versions, in this PR, we agreed to keep changes limited to the accounting scope.

Ticket link: https://www.odoo.com/web#id=2645653&model=project.task



opw-2645653

closes odoo/odoo#82580

Signed-off-by: William André (wan) <wan@odoo.com>