Make it harder to forget to submit targets/filters subform

@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.

Start by locating the firewall rules form and its target and filter subforms, then reproduce submitting text without clicking Add Target. Trace how those fields are handled on overall form submission and choose a warning behavior consistent with the existing UI. Done means users are alerted that target or filter input remains unsubmitted instead of silently losing it.