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 :)
To be clear, here's the scenario I'd like to accomplish with this change:
 1. User Alice is member of the Foo organization, but not entered into the admin list
 2. User Bob opens PR but is not whitelisted yet, so no build occurs
 3. Alice checks the PR and whitelists Bob or just requests a single build, this now works because Alice is a member of the Foo org.