Sending topic slug instead of topic ID for /attempt call.

There is a corresponding change on the server side to use topic_slug instead of
topic_id. This is needed before I merge the TopicRevision changes which
return the actual key ID for "id" when sending down the Topic rather
than the slug.

Test plan:
- Submitted a correct answer in power mode. Verified that the topic's slug was sent up.

Auditors: eater