Technically, this is one commit before the 1.52.0 tag to keep it a
pseudo-version.

Signed-off-by: Will Norris <[email protected]>

The "tailscale.com/golink" peercap includes a single "admin" bool field.
When set, this grants the user the ability to edit all links stored in
the system.

Update currentUser to return a simple user struct instead of just a bare
username. Rename checkLinkOwnership to canEditLink and change to a bool
return value.

Signed-off-by: Will Norris <[email protected]>