web: Add parameters to API requests for empty topic name support.

This commit adds `allow_empty_name: true` to the following
endpoints to receive empty strings as topic name when such
topics are available:
* `GET /messages`
* `GET /messages/{message_id}`
* `GET /messages/{message_id}/history`

Also, it adds `empty_topic_name` client capability for spectators.

These changes are part of the broader effort to enable
support for empty string as a topic name.

web/src/message_flags.ts

@@ -110,6 +110,7 @@ export function unstar_all_messages_in_topic(stream_id: number, topic: string):
             {operator: "topic", operand: topic},
             {operator: "is", operand: "starred"},
         ]),
+        allow_empty_topic_name: true,
     };
 
     void channel.get({