feat(networks): a network's DHCP leases on its page

- A DHCP leases card on the page of a network that serves DHCP: each
  lease's address, MAC or an IPv6 client's DUID, the host name the client
  sent, the VM whose card has that MAC, linked, and when it ends.
- A lease that is a fixed address is marked; any other in one of the
  network's DHCP subnets has a Make fixed button that fills in the fixed
  address dialog with its address, MAC or DUID and name.
- Leases that cannot be read are said on the card; the rest of the page
  stays. A network that does not run has none.
- The VMs on a network are read with the MACs of their cards on it, from
  the definitions already read.

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