Use the config.inc.php default_timezone in favor of first admin

[Luke1982]

Right now we use the timezone of the first found admin user when we are executing things like scheduled reports or workflows without a clear user context. I would like to alter this to use the default_timezone we have in the config.inc.php file so that it's clear what the 'fallback' is no matter what. The first admin user may change over time, you may forget to set a time zone for that user or something like that. I think using the config.inc.php setting would be less ambiguous.