Add the possibility to customize the remote user header name

Inspired by the discussion here https://github.com/jonaswinkler/paperless-ng/discussions/639#discussion-3242017 it is worthwhile to be able to customize the header name that is used for authentication as its name is not really standardized.

src/paperless/auth.py

@@ -33,5 +33,4 @@ class HttpRemoteUserMiddleware(RemoteUserMiddleware):
     """ This class allows authentication via HTTP_REMOTE_USER which is set for
         example by certain SSO applications.
     """
-
-    header = 'HTTP_REMOTE_USER'
+    header = settings.HTTP_REMOTE_USER_HEADER_NAME

src/paperless/settings.py

@@ -189,6 +189,7 @@ if AUTO_LOGIN_USERNAME:
     MIDDLEWARE.insert(_index+1, 'paperless.auth.AutoLoginMiddleware')
 
 ENABLE_HTTP_REMOTE_USER = __get_boolean("PAPERLESS_ENABLE_HTTP_REMOTE_USER")
+HTTP_REMOTE_USER_HEADER_NAME = os.getenv("PAPERLESS_HTTP_REMOTE_USER_HEADER_NAME", "HTTP_REMOTE_USER")
 
 if ENABLE_HTTP_REMOTE_USER:
     MIDDLEWARE.append(