New admin-only page at /servers-settings (templates/servers.html) lists
servers and creates new ones via POST /servers (ID/name/interface/
addresses/port, key pair generated server-side). Nav gets a "Servers"
link.
templates/users_settings.html gains a multi-select "Server Access"
field wired to the server_ids support added to create-user/update-user
in the previous commit, so admins can now actually assign non-admin
users to specific servers through the UI.