We now send a new user_topic event while muting and unmuting topics.
fetch_initial_state_data now returns an additional user_topics array to
the client that will maintain the user-topic relationship data.
This will support any future addition of new features to modify the
relationship between a user-topic pair.
This commit adds the relevent backend code and schema for the new
event.
Tim Abbott
Kartik Srivastava
Zixuan James Li
Anders Kaseorg