Note that this only works for people who are currently logged in.
Folks that log in after you may pick up the old full name from
the message. (I'll address this in a separate commit.)
Most of the magic happens in message_live_update.update_avatar().
The prior code was buggy, as it was using person.id instead of
person.user_id, and it was not setting the image resolution.
When we change a stream name, we now use the stream id as the
key to find messages we need to live update. This eliminates
some possible race conditions from two users renaming a stream.
This commit introduces message_live_update.js.
The new call stack is this:
subs.update_subscription_properties
subs.update_stream_name
message_live_update.update_stream_name
message_list.update_stream_name