Fix READMEs and Changelogs not rendering on /libs/libname

[chapulina]

I could swear we already had this issue ticketed somewhere but I can't find it now 🤷‍♀️

After migrating to GitHub, our READMEs stopped being rendered on pages like this:

https://ignitionrobotics.org/libs/math

If you check the console, you'll see CORS errors like:

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://github.com/ignitionrobotics/ign-math/raw/master/Changelog.md. (Reason: CORS header ‘Access-Control-Allow-Origin’ does not match ‘https://render.githubusercontent.com’).
Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://github.com/ignitionrobotics/ign-math/raw/master/README.md. (Reason: CORS header ‘Access-Control-Allow-Origin’ does not match ‘https://render.githubusercontent.com’).
Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://github.com/ignitionrobotics/ign-math/raw/master/Changelog.md. (Reason: CORS request did not succeed)
Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://github.com/ignitionrobotics/ign-math/raw/master/README.md. (Reason: CORS request did not succeed).

[chapulina]

@ahcorde bravely fixed this issue on the server side 🎉