Many permissions changes

Mostly, use "Administrators" group instead of mdb/pigweed-gob-admin-acl.
Also grant "Googlers" marginally elevated permissions, and remove any
permissions to push directly to "refs/heads/*".

Change-Id: I1c74dd5b42274e0fc516fd736cda3af224860fdb
Reviewed-on: https://bluetooth-review.git.corp.google.com/c/All-Projects/+/1000
Reviewed-by: Oliver Newman <olivernewman@google.com>