fix binascii.Error with reverse proxy bearer token

When an authenticating reverse proxy (e.g. Keycloak Gatekeeper) adds a Bearer token in the Authorization header, every request fails with HTTP status code 500.  The corresponding error in the logs is: binascii.Error: Incorrect padding. 
Despite "reverse_proxy_header_login" is enabled, calibre-web tries first to base64decode the bearer token and fails. This patch just reverses the order in which the authentication methods  are checked.

cps/web.py

@@ -124,12 +124,6 @@ def load_user(user_id):
 
 @lm.request_loader
 def load_user_from_request(request):    
-    auth_header = request.headers.get("Authorization")
-    if auth_header:
-        user = load_user_from_auth_header(auth_header)
-        if user:
-            return user
-
     if config.config_allow_reverse_proxy_header_login:
         rp_header_name = config.config_reverse_proxy_login_header_name
         if rp_header_name:
@@ -139,6 +133,12 @@ def load_user_from_request(request):
                 if user:
                     return user
     
+    auth_header = request.headers.get("Authorization")
+    if auth_header:
+        user = load_user_from_auth_header(auth_header)
+        if user:
+            return user
+
     return