We should now rename set_muted_topics to set_user_topics as with
the new user_topic event, there will be various types of user-topic
configurations to handle other than just muting topics.
Since we are replacing muted_topics with user_topics, the web app
should now be using the user_topics page_param to construct the
muted_topics map. Also, the UI should now use the user_topic event
instead of muted_topics to handle topic updates.
This updates user_topics.js to use the new user_topics page_param to
initialize the muted topics map. This also replaces the "muted_topics"
clause in server_events_dispatch.js with a "user_topic" clause.
As we plan to move towards using `user_topics` instead of
`muted_topics`, this helper method will be used to set an
individual `user_topic` event in the corresponding data
structure in the web app.
Kartik Srivastava