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.

[charliepark]

I'm not totally against disabling the form if there's stray data. "You have an unsaved {target|host} filter; save or clear it to create this firewall rule."

[david-crespo]

Yeah, on second thought it’s rare enough that it doesn’t feel like too much friction.

[david-crespo]

As noted in #2634 (comment), we are hoping to fix this by eliminating the subforms and making the minitable editable directly instead. That PR (which doesn't exist yet) can close this issue.