Allow admins full ssh access

This feature allows anyone who is an admin in gitlab normal SSH
access, including starting a tty session; in effect causing
gitlab-shell to become transparent to admins.

I wrote this so capistrano will continue to work without major
kludges (like sudo).

If GitLab isn't running, then gitlab-shell will deny access
to admins (unfortunately).

This feature requires a new GitLab internal api call, "admin", to
check if a given SSH key belongs to an unblocked admin.

When I made these changes, I also replaced `system()` calls with
`Kernel::exec()` because there is no need to keep the ruby process
around once we've authenticated the user.