Implement integration with Salsa Single Sign-On

As a Debian developer, I want to be able to login on debusine.debian.org with SSO provided by salsa.debian.org.

As a system administrator, I want to make sure that only Debian developers have accounts on debusine.debian.org, non-DD that have accounts on salsa.debian.org should not be able to login (or at least should not have any privileges, i.e. no write rights at all, no access to any restricted data).

Design concerns

We probably want to make something generic that can be leveraged by other Django applications like distro-tracker. Maybe it ends up as a small side project.

We can possibly get some inspiration from https://salsa.debian.org/nm-team/nm.debian.org/-/tree/master/signon