auth: only use X- headers instead of REMOTE_ADDR if explicitly told so in remote_addr_header

Before, X-Forwarded-For (and others) headers would *always* be trusted blindly,
also in setups without a proxy server. It would thus in some cases be
possible for users to fake their IP, and thus potentially be possible to bypass
IP restrictions configured in Kallithea.

Fixed by making it configurable which WSGI environment variable to use for the
remote address. Users can configure remote_addr_header to for example
HTTP_X_FORWARDED_FOR instead of using the default REMOTE_ADDR.

This change is a bit similar to what is going on in the https_fixup middleware,
but is doing a bit more of what for example is happening in similar code in
werkzeug/middleware/proxy_fix.py .