CSRF protection support

[miguelgrinberg]

Fixes #321

CSRF protection based on the Sec-Fetch-Site and Origin headers sent by browsers.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (053b8a8) to head (623e567).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #335   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        16    +1     
  Lines         1607      1653   +46     
  Branches       288       299   +11     
=========================================
+ Hits          1607      1653   +46     

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