[mod] limiter: trusted proxies (#4911)

Replaces `x_for` functionality with `trusted_proxies`. This allows defining
which IP / ranges to trust extracting the client IP address from X-Forwarded-For
and X-Real-IP headers.

We don't know if the proxy chain will give us the proper client
address (REMOTE_ADDR in the WSGI environment), so we rely on reading the headers
of the proxy before SearXNG (if there is one, in that case it must be added to
trusted_proxies) hoping it has done the proper checks. In case a proxy in the
chain does not check the client address correctly, integrity is compromised and
this should be fixed by whoever manages the proxy, not us.

Closes:

- https://github.com/searxng/searxng/issues/4940
- https://github.com/searxng/searxng/issues/4939
- https://github.com/searxng/searxng/issues/4907
- https://github.com/searxng/searxng/issues/3632
- https://github.com/searxng/searxng/issues/3191
- https://github.com/searxng/searxng/issues/1237

Related:

- https://github.com/searxng/searxng-docker/issues/386
- https://github.com/inetol-infrastructure/searxng-container/issues/81

searx/webapp.py

@@ -57,7 +57,7 @@ from searx import (
 
 from searx import infopage
 from searx import limiter
-from searx.botdetection import link_token
+from searx.botdetection import link_token, ProxyFix
 
 from searx.data import ENGINE_DESCRIPTIONS
 from searx.result_types import Answer
@@ -1391,6 +1391,7 @@ def static_headers(headers: Headers, _path: str, _url: str) -> None:
         headers[header] = str(value)
 
 
+app.wsgi_app = ProxyFix(app.wsgi_app)
 app.wsgi_app = WhiteNoise(
     app.wsgi_app,
     root=settings['ui']['static_path'],