Add injectable username query param

username=<currently_logged_in_username_or_id>

We can use to invalidate sessions in Opstrace if the value changes - it doesn't matter what field we use (username or id). So when someone logs out of GitLab and logs in as a different user, they'll also be re-authenticated in Opstrace as the new user (because we'll invalidate the current session).

Reach out to @mappelman for more details