session: Override PHP default for session lifetime

The PHP.ini default is 1440 seconds (24 minutes). This should be configured
to something significantly higher so that the settings in the admin panel
concerning session timeouts are relevant.

Ideally, the settings from the control panel would be used, but currently
there is an inter-dependency between session and config startups.
Merge pull request #1111 from greezybacon/issue/860

session: Override PHP default for session lifetime

Reviewed-By: Peter Rotich <peter@osticket.com>