refactor: Add compare_function to buddy_list.

We were passing this in before, but having it as
a data member reinforces the idea that we'll want
this to be a first-class concept in the list, since
we depend on ordering for various things.

static/js/activity.js

@@ -218,7 +218,6 @@ exports.insert_user_into_list = function (user_id) {
     buddy_list.insert_or_move({
         key: user_id,
         item: info,
-        compare_function: buddy_data.compare_function,
     });
 
     exports.update_scrollbar.users();