The choice regarding which forms should or should not trigger a warning
is subjective. I tried to be consistent and not warn about forms that:
- run an action, rather than edit data: search, send an email.
- delete data: a warning about losing data would be confusing
Note that forms on sign-in pages were already ignored (using a selector,
rather than an explicit class on the form element).
Fixes #3698.
2dc6f15eca