YDA-6661: Move Flask session data to dedicated dir

[stsnel]

Move EUS and portal Flask session data to dedicated directory outside the /tmp filesystem.

The Apache systemd unit files on Ubuntu and EL have PrivateTmp enabled, so if we store session data in /tmp, the web server loses its session data every time the process is restarted or (on EL) reloaded. This can cause user sessions to be terminated unintentionally, e.g. when logs are rotated.