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 Loading Dashboard with no data fails with "Application Error" #24)
• Removes unnecessary Promise layer in test code (when mocking responses via fetch.mockResolvedValue)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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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