* Fix error session already started
Got this error when going on eg `index.php?module=PrivacyManager&action=privacySettings&idSite=1&period=week&date=2019-09-20` and a session was started by another tool. That page uses NONCE which then uses SessionNamespace which then wants to start the session even though it was already started.
I don't think I can write a test for it but need to see.
* add test
* If session was already started, do not start it again
refs https://github.com/matomo-org/matomo/issues/12963
refs https://forum.matomo.org/t/an-error-occurred/32500
* Update Session.php
* Update bootstrap.php
* add test
