admin: make ProlongationRequest.status readonly

Otherwise one could "accept" (or "deny") the request in the admin
without triggering anything (like creating the accepted prolongation).

itou/approvals/admin.py

@@ -575,7 +575,7 @@ def denied_proposed_actions_explanation(self, obj):
         return obj.deny_information.proposed_actions_explanation
 
     def get_readonly_fields(self, request, obj=None):
-        fields = ProlongationCommonAdmin.readonly_fields + ("processed_at", "processed_by", "prolongation")
+        fields = ProlongationCommonAdmin.readonly_fields + ("processed_at", "processed_by", "prolongation", "status")
         if not obj:
             return fields