The create-network form only checked the characters of its subnets, so
192.0.2.0/33 or 999.0.2.0/24 passed and network_size raised a
ValueError: the user got a server error instead of a message. So did a
subnet too small for the gateway, DHCP range and broadcast address that
network_size derives (0.0.0.0/32, 255.255.255.255/32), and 192.0.2.0/31
or a bare address gave an inverted DHCP range.
Each subnet field now parses its value as a network of its IP version
and refuses it with a field error otherwise. An IPv4 prefix must be /30
or shorter; the IPv6 prefix must be /64, which the view checked after
the form and now the form does.
