Proxito: add CORS headers only on public versions (#10737)
* Proxito: add CORS headers only on public versions
Add `Access-Control-Allow-Origin: *` and `Access-Control-Allow-Methods: OPTIONS,
GET` HTTP headers when serving a PUBLIC version.
This is required for DocDiff addon to be able to download the original version
of the page and compare the DOMs to show the visual differences.
Closes https://github.com/readthedocs/addons/issues/93
* Merge conflict fixed
* Update readthedocs/proxito/middleware.py
* Update readthedocs/proxito/tests/test_headers.py
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Co-authored-by: Eric Holscher <25510+ericholscher@users.noreply.github.com>