* Introduce a hook to track active room
This hook simply keeps a state which tracks the index of the active room
in the list of rooms passed through props. This index will be recomputed
if the active rooms changes or if the list itself changed.
* Use hook in the view model
* Write tests
* Fix broken tests
Any UTs for vector-web layer components or functionality should go here.
This used to contain the UTs for notifications before they got moved to react-sdk.
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