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 I would really like to have this feature to.
We have over a hundred members in our github organisation and I really don´t want to have to whitelist them all manually.

Another option would be as @akoeplinger suggested, to simple allow all members of the organisation which owns the repo to start PR builds.