Dashboards: Fix issue causing crashes when saving new dashboard

[kaydelaney]

Maybe "avoids issue" is a better title. The underlying issue is here:

grafana/public/app/features/dashboard/components/SaveDashboard/SaveDashboardErrorProxy.tsx

Lines 32 to 36 in d624a5d

	useEffect(() => {
	if (error.data && proxyHandlesError(error.data.status)) {
	error.isHandled = true;
	}
	}, [error]);

Specifically line 34 where error.isHandled is set to true.

It seems like what happens is:

1. User creates new dashboard
2. User adds panel, edit view is opened
3. User hits save, drawer opens, then confirms save
4. POST request is made with dashboard json, is saved successfully
5. After save, tries to exit panel edit view
6. DashboardPrompt onHistoryBlock function is called.
7. hasChanges(dashboard, original) is called, but at this stage original still has version 0, while dashboard has version 1. Note that there is code in DashboardPrompt to update original when a DashboardSavedEvent is emitted, but I guess hasChanges gets executed first
8. Unsaved changes prompt appears, user hits save
9. POST request is made with the same dashboard json, error gets returned since it already exists
10. Line 34 in SaveDashboardErrorProxy occurs, causing the crash.

So with this PR the issue is sort of avoided by saying "ignore changes if all we're changing is a dashboard that hasn't been saved yet", but I guess a "real" fix would figure out how to update original in DashboardPrompt before it looks for unsaved changes.

Closes #77593

[dprokop]

Confirm - this is fixing the problem :) It's just some tests that will require updating.