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.

docs/configuration.rst

@@ -193,6 +193,16 @@ PAPERLESS_ENABLE_HTTP_REMOTE_USER=<bool>
 
     Defaults to `false` which disables this feature.
     
+PAPERLESS_HTTP_REMOTE_USER_HEADER_NAME=<strl>
+    If `PAPERLESS_ENABLE_HTTP_REMOTE_USER` is enabled, this property allows to 
+    customize the name of the HTTP header from which the username is extracted.
+    Values are in terms of
+    [HttpRequest.META](https://docs.djangoproject.com/en/3.1/ref/request-response/#django.http.HttpRequest.META).
+    Thus, the configured value must start with `HTTP_` followed by the 
+    normalized actual header name.
+    
+    Defaults to `HTTP_REMOTE_USER`.
+    
 .. _configuration-ocr:
 
 OCR settings

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(