Fix https github oauth

[flashlab]

@OzzieIsaacs After spending several hours digging the github oauth problem #1307 , I finally makes it work with https domains.
First, the nginx configuration is needed to properly proxy the real domain and protocol to Flask, following this example.
And pyopenssl model is required: pip3 install -U pyopenssl.
Finally, import ProxyFix to make flask properly identity https protocols.
Feel free to test here https://calibre.redbux.cn
Some useful links：
https://stackoverflow.com/questions/54278247/flask-dance-and-error-redirect-uri-mismatch
https://flask-dance.readthedocs.io/en/latest/proxies.html
singingwolfboy/flask-dance#188

[OzzieIsaacs]

I had time to doublecheck the PR.
By just using the example given in the Calibre-Web wiki, replacing the /calibre with / and adding ssl to the nginx configuration it worked out of the box. Because werkzeug is talking about security concerns in using the proxyfix function and it shall only be used if it is necessary, I reverted your PR. I didn't have to install anything additional. I've tested it with an up to date Manjaro Linux with nginx 1.17 ("mainline"), the ssl Certificates where signed by Let's encrypt