Explicit repo permissions for Sourcegraph organizations (as well as users)

Created by: dadlerj

Requested by https://app.hubspot.com/contacts/2762526/company/554338610

They were wondering if there was a way to enforce permissions at the group level (so they could add/remove people from groups to control access). We don't offer this, as far as I know (and I worry about the fact that Sourcegraph org members are allowed to add/remove others? I'm not sure if orgs have the proper security around them), so I assume this would require new development.

Curious what you think about this proposal @unknwon, and how difficult it would be to support? Not high priority yet, but wanted to get on your radar.