Refactor date selector + add heatmap unit tests

[sknep]

This PR finishes some work that was merged in partially complete last week. It more reliably watches changes to the selected date and surfaces that via a query parameter in the URL, allowing users to link to specific days. By default, the library page opens with yesterday's data (note that our sample data only covers May 2022 right now), although each date selection change now updates the browser history. This removes the need for storing a selectedDate in shared state across the app, as the selected date is now available from the router and is passed in to the library page view as a prop. This also updates the Fetch Data wrapper to consume the date as a prop passed in from parent, rather than shared in state, enabling simultaneous use with different start dates. Finally, this PR adds unit test coverage for the two heatmaps, as that work was unfinished when the previous branch merged.

[github-actions[bot]]

🦙 MegaLinter status: :warning: WARNING

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[anagradova]

I just went through the changes and code, looks good as far as I can tell.