Fix Django CSRF protection behind proxy automatically

[undergroundwires]

After migrating Seafile 11 running Django 4.0, users using proxies start getting Origin checking failed - XX does not match any trusted origins. errors.

Handling this is documented in Server FAQ.

Setting CSRF_TRUSTED_ORIGINS = ["{url}"] in {data_dir}/seafile/conf/seahub_settings.py solves the issue.

It would be nice that the docker instance configures this automatically for a seamless experience so not everyone spends hours on debugging this like me.

It would be nice to introduce a variable like PROXY_ROOT_URL or PROXY_BASE_URL_WITH_SCHEME. So in this code it can set CSRF_TRUSTED_ORIGINS = ["{PROXY_ROOT_URL}"] (if PROXY_ROOT_URL is defined) to the URL. Or just use {proto}://{domain} utilizing FORCE_HTTPS_IN_CONF without introducing any new configuration.

Solves #347, and a lot of other StackOverflow, forum GitHub issues.