[FIX] website: Display duplicate login error message

When you try to create two users with the same login, the shown error message is
the Postgresql integrity error instead of a nice (translated) validation error.
This happens since bf88f3e3 which changed the way error messages are translated.

The reason is the manually created unique index `res_users_login_key_unique_website_index`.
The error handling and translation mechanisms don't know it exists.
The index was created for performance reasons in commit b5a12b4a where the
original python constraint was replaced with this custom SQL index.

In this commit, the python constraint is re-introduced to remove the index hack,
but the implementation now uses a single SQL query, which should be quite fast.

closes odoo/odoo#37901

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