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_fetch.ts

@@ -59,6 +59,7 @@ type MessageFetchAPIParams = {
     num_after: number;
     client_gravatar: boolean;
     narrow?: string;
+    allow_empty_topic_name: boolean;
 };
 
 let first_messages_fetch = true;
@@ -329,6 +330,7 @@ function get_parameters_for_message_fetch_api(opts: MessageFetchOptions): Messag
         num_before: opts.num_before,
         num_after: opts.num_after,
         client_gravatar: true,
+        allow_empty_topic_name: true,
     };
     const msg_list_data = opts.msg_list_data;