Use config URL instead of THISPAGE

THISPAGE is unreliable for CLI cron calls. It was historically used so the
admin could figure out which file/page generated the error. But with
backtrace support we no longer need it.
Merge pull request #412 from protich/issue/baseurl

Use config URL instead of THISPAGE

Reviewed-By: Jared Hancock <jared@osticket.com>