deletion: Add support for bulk message deletion events.

This is designed to have no user-facing change unless the client
declares bulk_message_deletion in its client_capabilities.

Clients that do so will receive a single bulk event for bulk deletions
of messages within a single conversation (topic or PM thread).

Backend implementation of #15285.

zerver/views/home.py

@@ -191,6 +191,7 @@ def home_real(request: HttpRequest) -> HttpResponse:
 
     client_capabilities = {
         'notification_settings_null': True,
+        'bulk_message_deletion': False,
     }
 
     register_ret = do_events_register(user_profile, request.client,