Support importing networks by name (#113)

- E.g. `terraform import unifi_network.mynetwork name=LAN`
- Bumps cidr prefix in tests to avoid:
    Call to function "cidrsubnet" failed: prefix extension of 4 does not
    accommodate a subnet numbered 18.

docs/resources/network.md

@@ -88,4 +88,7 @@ terraform import unifi_network.mynetwork 5dc28e5e9106d105bdc87217
 
 # import from another site
 terraform import unifi_network.mynetwork bfa2l6i7:5dc28e5e9106d105bdc87217
+
+# import network by name
+terraform import unifi_network.mynetwork name=LAN
 ```