Add support for authentication proxies (#321)

* add support for auth proxies

* Improve docs

bookmarks/middlewares.py

@@ -0,0 +1,6 @@
+from django.conf import settings
+from django.contrib.auth.middleware import RemoteUserMiddleware
+
+
+class CustomRemoteUserMiddleware(RemoteUserMiddleware):
+    header = settings.LD_AUTH_PROXY_USERNAME_HEADER