[FIX] mail, portal, account: clean up portal attachment

Follow up of 61de1c26

- Remove the dangerous **kw from the route in favor of specifying useful args.

- Use the existing method to link attachments to message instead of duplicating
the logic in an incomplete manner.

- Fix an issue where the actual email would not be sent if the message was empty
but an attachment was defined.

- Ensure the send attempt is done immediately instead of after the cr commit.
The goal is to show potential error messages correctly to the user with the
crash manager instead of a generic error page.

This also increases the send timeout and prevents the error in the first
place in most situations.

closes odoo/odoo#35263

Signed-off-by: Christophe Simonis <chs@odoo.com>