Make it harder to forget to submit targets/filters subform

[david-crespo]

@iliana reports (and I think @charliepark have discussed this before) that it feels like the firewall rules form is just ignoring your target because you typed it in but forgot to click Add Target. There is probably something we can do about this. It's probably too much to refuse to submit the overall form if there's unsubmitted stuff in the target or filter field, but we can probably warn the user somehow.