Flesh out email templating

Mostly WIP porting the room name calculation logic from the web client so our room names in the email mirror the clients.

synapse/python_dependencies.py

@@ -46,7 +46,7 @@ CONDITIONAL_REQUIREMENTS = {
         "netaddr>=0.7.18": ["netaddr"],
     },
     "email.enable_notifs": {
-        "Jinja2": ["Jinja2"],
+        "Jinja2>=2.8": ["Jinja2>=2.8"],
     },
 }