Disable trusted users by default

[Cole Helbling]

The current Darwin build machine is restarted and erased relatively frequently, so the sandboxing isn't as much of a worry anymore

• Add "enable_trusted_users": true/false, toggle to config.public.json
• Skip ACL stuff if that is false
• Update README
• Close all trusted user PRs

We don't want to remove the ACL stuff, just in case... It's easy to toggle back on if anything unexpected happens.

[Graham Christensen]

restarted

-> restarted and erased