SYN-267 Add a fallback rule as an explicit server default rule and make the default dont-notify so you effectively have a "notify for everything else" switch you can turn on and off.

synapse/push/baserules.py

@@ -32,6 +32,8 @@ def make_base_rules(user, kind):
 
     if kind == 'override':
         rules = make_base_override_rules()
+    elif kind == 'underride':
+        rules = make_base_underride_rules()
     elif kind == 'content':
         rules = make_base_content_rules(user)
 
@@ -98,3 +100,16 @@ def make_base_override_rules():
             ]
         }
     ]
+
+
+def make_base_underride_rules():
+    return [
+        {
+            'rule_id': 'global/underride/.m.rule.fallback',
+            'conditions': [
+            ],
+            'actions': [
+                'notify',
+            ]
+        },
+    ]