fix(FormEditor): form cache not being cleared

[maxpatiiuk]

Because of stale state, the cache for the form that was edited most recently wasn't getting cleared.

Changed to ref, since using state here didn't make sense in the first place (state isn't used in render). I used state because at the time I followed idea that we should use state whenever possible, and ref only when must, because state would be cleaner and less bug prone. However, today I belive that each has it's place - use state when you need to use the value in the render or dependency array - use ref otherwise

Fixes #1977

[pashiav]

Instructions for testers? @maxpatiiuk So far the cache doesn't seem to be clearing on Chrome.

[maxpatiiuk]

1. Open a form definition in app resources and make an edit
2. Without reloading the page, open that form and see that changes are visible
3. Reload the page and see that changes are visible (this is different code pathway than 2)
4. If 2 or 3 does not work, open incognito browser tab, and make sure changes are visible (if not, then there is a mistake in step 1 - perhaps you edited a form that is overwritten at a different level)

edit: I tested this again just now and it worked for me

[emenslin]

I just tested this and I was able to see changes without having to reload the page so it seems like it is working.