Move proxy header check to auth-requiring endpoints

The main drive behind this is to allow us to enable http healthchecks on
the `/_status` endpoint. The healthcheck requests are happening directly
on the instances without going to the proxy to get the header properly
set.

In any case, endpoints like `/_status` should be generally accessible by
anything without requiring any form of authorization.

app/authentication/auth.py

@@ -1,6 +1,7 @@
 from flask import request, _request_ctx_stack, current_app, g
 from notifications_python_client.authentication import decode_jwt_token, get_token_issuer
 from notifications_python_client.errors import TokenDecodeError, TokenExpiredError, TokenIssuerError
+from notifications_utils import request_helper
 from sqlalchemy.exc import DataError
 from sqlalchemy.orm.exc import NoResultFound
 
@@ -48,6 +49,8 @@ def requires_no_auth():
 
 
 def requires_admin_auth():
+    request_helper.check_proxy_header_before_request()
+
     auth_token = get_auth_token(request)
     client = __get_token_issuer(auth_token)
 
@@ -59,6 +62,8 @@ def requires_admin_auth():
 
 
 def requires_auth():
+    request_helper.check_proxy_header_before_request()
+
     auth_token = get_auth_token(request)
     client = __get_token_issuer(auth_token)