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 :)
How would you remove them given this change? Once you've whitelisted them by adding a comment, it's a bit opaque as to who's been whitelisted.