Re-enable ACL's for Teamfolders

[jospoortvliet]

For several organizations, ACL's are really a key feature for Team folders. We should re-enable this for 35.0.1.

We disabled it to keep things simpler, but it would mean quite some orgs can't use 35's new Team folder concept. I would thus suggest simply turning it on again.

NOTE, quoting @artonge

Also, I can see an issue coming with ACL + .system folder in which we store the Teams data, which means, everybody should be able to read and write to it.

So we have to solve that first. Let's see if that is doable for .1

[hefftich]

PR for changes in groupfolders: #5123