Proxito: update CORS settings (#10751)
* Proxito: update CORS settings
- Only add CORS headers on community site
- Explicit host on `Access-Control-Allow-Origin`
- Only query the database if the host ends with `RTD_EXTERNAL_VERSION_DOMAIN`
- Add more tests
Continuation of #10737
* Add `Vary: Origin` header
* Use Django internals to patch `Vary` header.
* Use the `Origin` header from request to check for the allowed domain
* Update tests to use `origin` header
* Allow cross-origin requests for public versions' docs
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Co-authored-by: Santos Gallegos <stsewd@proton.me>
