Cleaner approach to HeaderMixin

[kennethlove]

This PR introduces a backwards-compatible change to the HeaderMixin. Our existing method works but feels kind of heavy-handed. This approach is more in-line with Django's design, IMO, and doesn't feel as blunt.

Unfortunately, it only works if render_to_response is ultimately called, so I had to leave in the old approach as well. Maybe we should split it up into a new mixin?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.77%. Comparing base (df847c9) to head (2f1a4ec).
⚠️ Report is 23 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #285      +/-   ##
==========================================
+ Coverage   97.72%   97.77%   +0.05%     
==========================================
  Files           8        8              
  Lines         484      495      +11     
  Branches       68       69       +1     
==========================================
+ Hits          473      484      +11     
  Misses          9        9              
  Partials        2        2              

Flag	Coverage Δ
unittests	97.77% <100.00%> (+0.05%)	⬆️

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Files with missing lines	Coverage Δ
braces/views/_other.py	100.00% <100.00%> (ø)

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