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.
The MAC field's pattern ([0-9/:]+) had no hex letters, so the browser
refused nearly every MAC address (52:54:00🆎cd:ef) and no IPv4 fixed
address could be added from the page. The host name field of both
dialogs ([a-zA-Z0-9_]+) refused names such as web-01 or db.lan. The MAC
takes hex digits and ":", the name letters, digits, "_", "." and "-";
the server already escapes both into the network XML.
