When no site provided on query string, redirect w/ with valid site specified

[benvinegar]

This has a couple of benefits:

• After redirect, doesn't wait for getSitesOrderedByHits to finish before querying data (no blocking before fetching data; faster)
• Avoids situations where different URLs could generate the same document (better consistency IMO), i.e. we were loading a default site at /dashboard and this would generate the same document as /dashboard?site=default
• No longer throws when there is no data in the AE dataset (fixes #24)
• Removes unnecessary Promise layer in test code (when mocking responses via fetch.mockResolvedValue)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (2265b13) 72.38% compared to head (01560b6) 72.51%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #35 +/- ## ========================================== + Coverage 72.38% 72.51% +0.12% ========================================== Files 18 18 Lines 1934 1943 +9 Branches 115 119 +4 ========================================== + Hits 1400 1409 +9 Misses 528 528 Partials 6 6 ```

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