fix crash when canceling alert association

[panentheos]

Asana task: bug: UI crash when canceling out of alert linking on PA message form

This fixes an issue where starting an alert linkage and then canceling would crash the UI. The underlying issue was that the sub-form was directly updating some state on the main form, and so canceling would leave things in an inconsistent state. The fix isolates the sub-form with its own state, which is only applied to the main form when the "Apply" button is pressed.

This also demonstrates an approach to prop handling that I think is generally useful: Rather than passing setters down to sub-components and requiring them to update things directly, it's sometimes nicer to think about passing contextual callbacks (e.g. onApply, onSelectAlert) and performing the actual state changes higher up in the tree. This naturally centralizes state management (which is a good thing), and also tends to reduce the amount of "wiring" required between components.

[digitalcora]

I'd agree this form of control flow makes the logic easier to follow and harder to make mistakes with. Approving so we can get the fix out, but @cmaddox5 and @PaulJKim have had much more involvement with this code, so everyone should take a look!