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 :)
We would have to remember to add new users in the organization to the admin list and e.g. remove them when they leave. So yeah, it would be doable but is quite inconvenient (though I understand your concern with config option bloat).