Feature/cancel initial subscriptions

Add the cancel_subscription workflow that allows for taking a subscription from INITIAL to TERMINATED.

Since it's a termination workflow, we can't change this from showing up differently (they're forcefully all called "Terminate router" in this case). But only one is available to run, so I'd say this is acceptable.

Also, I added some subtile warnings to prevent accidentally running this.

docs/source/glossary.rst

@@ -3,6 +3,9 @@ Glossary of terms
 
 .. glossary::
 
+  AAI
+    Authentication and Authorisation Infrastructure
+
   API
     Application Programming Interface
 
@@ -23,9 +26,15 @@ Glossary of terms
   CRUD
     Create, Read, Update, Delete
 
+  DNS
+    Domain Name System
+
   FQDN
     Fully Quantified Domain Name
 
+  GAP
+    The GÉANT Automation Platform
+
   GSO
     GÉANT Service Orchestrator
 
@@ -42,6 +51,9 @@ Glossary of terms
   ISO
     International Organisation for Standardisation
 
+  JSON
+    JavaScript Object Notation
+
   LAG
     Link Aggregation: a bundle of multiple network connections.
 
@@ -69,6 +81,3 @@ Glossary of terms
 
   WFO
     `Workflow Orchestrator <https://workfloworchestrator.org/>`_
-
-  AAI
-    Authentication and Authorisation Infrastructure