Advertise server-wide message deletion capability.

This change allows Session clients to check whether the SOGS supports
"Ban from Server and Delete All".

This is a follow-up to oxen-io#167, which added
server-wide message deletion capabilities. As this change has not been
released, few SOGS support this useful capability, and clients cannot
know in advance whether the deletion request will be honored.

Ideally, clients should let users know if this feature is not supported.
This may be required from the official Desktop client in
session-foundation/session-desktop#1772.

Author: gravel

sogs/model/__init__.py

@@ -15,6 +15,7 @@
 capabilities = {
     'sogs',  # Basic sogs capabilities
     'reactions',  # Reactions, added in 0.3.1
+    'server-ban-and-delete',  # Support "Ban from Server and Delete All"
     # 'newcap',  # Add here
 }