fix(alerts): ensure hiding works correctly and alerts are not re-added [DHIS2-15438]

[HendrikThePendric]

This PR fixes a bug that was observed in the maps-app where multiple info alerts were added at approximately the same time. These alerts would then also hide at approximately the same time, but one alert would finish first and this would cause the other one to remount and restart its appear/show/hide lifecycle.

It took a long time to debug this issue and I found a few things wrong:

• It turned out that on occasion the alertStackAlerts was an empty array when it should have been populated. In the next render cycle, the array would have the expected alerts again, but because the array was empty during the previous render cycle, everything was remounting all the time.
• The fact that both the alerts-array from the alerts-manager and the local alerts array were stateful/reactive and because the latter was updated in response to changes in the former, was causing a double render all the time. This was making the problem a lot more difficult to reason about, and also wasteful. The new hook doesn't have a local reactive state for the alerts, it will only trigger a single re-render when the alerts from the alerts-manager change, or when one of the alerts is done hiding.

After fixing that empty-array and double render problem I realised I had broken some preexisting functionality: As I explain here, there is a fundamental difference between how alerts hide "naturally" VS programatically. I had already catered for this in the original implementation, but broke that while fixing the initial bug. I found this problem by chance: there were no test cases that broke. So I decided to add a test for this as well.

[dhis2-bot]

:tada: This PR is included in version 11.6.1 :tada:

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[dhis2-bot]

:tada: This PR is included in version 12.0.0-alpha.3 :tada:

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