[5.8] Extract session saving to a method

[jasonvarga]

This PR simply moves the saving logic into a method so we can override without needing to override the entire handle method.

Our use case:

We have a feature in our app that lets you track how long is left in your session from a specific route.

Every request will add a last_activity timestamp to the user's session.

On one route, we want to check the last_activity timestamp without extending the session (which, normally every request would do).

Our SaveSession middleware would end up looking like this:

protected function saveSession()
{
    if (request()->route()->named('statamic.cp.session.timeout')) {
        return;
    }

    session()->put('last_activity', now()->timestamp);

    parent::saveSession();
}