Add group ownership to links (either manually in-app or groups defined in Access Controls)

[DAcodedBEAT]

Currently each link is owned by a single user, which means that this user needs to always be responsible for managing this link.
It would be swell to make ownership for a link delegated to multiple users (either manually in-app or groups defined in Access Controls in Tailscale).

[willnorris]

We recently added support for making certain users admins that have permission to edit any link. I forgot to actually document that, but have just now done so. For now, this makes those users an admin of the whole golink instance. We could look at doing something more granular in the future so that you can delegate access on a per-link basis, but we wanted to start with something simple that should accommodate a lot of the use cases we've heard about.