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 :)
@akoeplinger Why do you want this behavior? Or rather, why can't you just edit the job and add the user as an admin? I see what you are trying to do here, but there are already a (large) number of configuration options, and I'd rather not introduce more unless necessary.