auth: only use X- headers instead of wsgi.url_scheme if explicitly told so in url_scheme_header - drop https_fixup setting

Before, several X- headers would be trusted to overrule the actual connection
protocol (http or https) seen by the Kallithea WSGI server. That was mainly
when https_fixup were set, but it incorrectly also kicked in if https_fixup or
use_htsts were configured. The ambiguity of which headers were used also made
it less reliable. The proxy server not only had to be configured to set one of
the headers correctly, it also had to make sure other headers were not passed
on from the client. It would thus in some cases be possible for clients to fake
the connection scheme, and thus potentially be possible to bypass restrictions
configured in Kallithea.

Fixed by making it configurable which WSGI environment variable to use for the
protocol. Users can configure url_scheme_header to for example
HTTP_X_FORWARDED_PROTO instead of using the default wsgi.url_scheme .

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 .

The semantics of the old https_fixup were unsafe, so it has been dropped.
Admins that are upgrading must change their configuration to use the new
url_scheme_header option.