IpNet firewall targets and host filters still broken

[plotnick]

#1127 was mostly fixed by #1161: Ip rule targets now work, and the action & direction are properly selected when editing existing rules (contra the PR comment). However, IpNet types for both targets and host filters are broken; trying to add a rule with either results in an error like this:

unable to parse JSON body: unknown variant `ipNet`, expected one of `vpc`, `subnet`, `instance`, `ip`, `ip_net` at line 1 column 1403

Seems like a camelCase vs. snake_case issue.

Also, when entering a target IP address, a spurious warning that the target "must start with a lower-case letter" is signaled.