This URL is used when user access the authentication domain without providing
the 'redirect' query parameter. In that case, Authelia does not know
where to redirect the user.
If the parameter is defined, Authelia can redirect the user to a default page
when no redirect parameter is provided.
When user is already authenticated and tries to access the authentication
domain, the "already logged in" page is rendered and it now tells the user he
is to be redirected in few seconds and uses this URL to redirect.
This parameter is optional. If it is not provided, there is only a notification
message at the end of the authentication process, as before, and the user is
not redirected when visiting the authentication domain while already
authenticated.
Example 1: The user has validated first factor when accessing a service
protected by basic auth. When he tries to access another service protected
by second factor, he is redirected to second factor step to complete
authentication.
Example 2: The user has already validated second factor. When he access auth
service, he is redirected either to /loggedin page that displays an "already
logged in" page or to the URL provided in the "redirect" query parameter.
