fix: registerUser request body is invalid (#4002)

I found two issues:
1. This `options` object is being passed down to fetchData,
 where it is deconstructed by the "data" attribute, not body.
2. the `data` object is being transformed into JSON downstream,
 thus the stringification done here makes for a string json object only

Signed-off-by: auyer <rafael@rcpassos.me>

web/services/chat-service.ts

@@ -34,7 +34,7 @@ class ChatService {
       headers: {
         'Content-Type': 'application/json',
       },
-      body: JSON.stringify({ displayName: username }),
+      data: { displayName: username },
     };
 
     const response = await getUnauthedData(URL_CHAT_REGISTRATION, options);