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 :)
@valdisrigdon Another option I could live with would be to always allow org members to trigger PR builds but NOT whitelist users (so whitelisting is allowed only by admins). This would remove the need for a config option.