Add custom warning message when leaving changing Data Model (segments and metrics) (#34398)

* Rename component to match the filename

* Fix typo

* Replace useBeforeUnload with LeaveConfirmationModal in SegmentForm

* Replace useBeforeUnload with LeaveConfirmationModal in MetricForm

* Update setup of MetricForm tests to allow testing navigation

* Update setup of SegmentForm tests to allow testing navigation

* Use proper history in MetricForm

* Refactor SegmentForm test setup to be consistent with the one of MetricForm

* Make MetricForm and SegmentForm tests consistent

* Add tests for not showing custom warning

* Add tests for showing custom warning when leaving with unsaved changes

* Extract FORM_URL

* Remove redundant asyncs and awaits

* Rename MetricForm.unit.spec.tsx & SegmentForm.unit.spec.tsx to MetricApp.unit.spec.tsx & SegmentApp.unit.spec.tsx and move them near the components they're testing

* Improve a11y

* Add missing endpoint mocks

* Improve selectors thanks to proper a11y

* Add tests for not showing warning modal when saving changes

* Convert CreateMetricForm to a functional component

* Convert CreateSegmentForm to a functional component

* Convert UpdateMetricFormInner to a functional component

* Convert UpdateSegmentFormInner to a functional component

* Disable LeaveConfirmationModal on scheduled navigation

* Setup metrics & segments POST endpoints

* Properly mock metric and segment creation

* Lift LeaveConfirmationModal from SegmentForm to SegmentApp

* Lift LeaveConfirmationModal from MetricForm to MetricApp

* Sort props

* Get rid of useCallbackEffect usage in MetricApp

* Get rid of useCallbackEffect usage in SegmentApp