Add option to allow members of whitelisted organizations to whitelist users/trigger PR tests

This adds a config option to allow members of whitelisted organizations to effectively behave like they were entered into the admin list, which is more practical for organizations with many members.

We are using this at the Mono project since a few days and it seems to work fine :)
Is this the correct way? My config option only applies on the Job config page, so it would throw a nullpointerexception when saving the global config.