reinstate staged configuration in panel data model as per data model requirements

architecture-docs/panel-data-model.mmd

@@ -0,0 +1,24 @@
+---
+title: Data model of sample web application
+---
+erDiagram
+    operator {
+        string username
+        string password_hash
+    }
+    deployment {
+        json deployed_configuration
+        option[string] staged_configuration
+        option[string] version
+    }
+    backup["back-up"] {
+        string bucket
+        string endpoint
+    }
+    keypair {
+        string access_key
+        string secret_key
+    }
+    operator ||--o{ deployment : has
+    deployment ||--o{ backup : has
+    backup ||--|{ keypair : authorises