Replace Registered Users with repo-discuss

Try to combat some of the change and voting spam by requiring membership
in the repo-discuss [1] group.

For Googlers to retain the current permissions assign the permissions to
mdb/all-users which is the group that represents all Googlers.

[1] https://groups.google.com/u/1/g/repo-discuss

Change-Id: Ic3fbaa7956755c3a236e489d4a89dede550a32d0