feat(v2): warn public form filler when form has changed on server-side

[karrui]

Problem

As originally raised by @mantariksh in #3457 (comment), we should not refresh the public form when the server state has changed. This may cause users to lose their currently filled in form fields. Instead, we should display a warning that form state has changed and the submission may fail due to such changes.

This PR implements that warning and prevents server side form changes from refreshing the current loaded form.
This is done by storing the initially fetched form and never changing it, whilst displaying a toast if the newly fetched form does change.

Possible future enhancements

The current methodology does a deep comparison between the current "cached" form and the form retrieved from the server, which may be inefficient. I don't think it will negatively impact responsiveness though, since the effect will not cause any rerenders -- even if a toast is shown a rerender is not triggered.

In the future, we may consider returning the _v prop when we retrieve the public form, a prop MongoDB exposes that declares the current version of the form, and compare that against the "cached" version for a simpler check.

Screenshots