refactor: Stop using page_params.realm_default_streams.

We now only use `page_params.realm_default_streams` during
initialization, and then after that we use `stream_data`
APIs to get default stream ids and related info.  (And
for the event that replace the data, we just update our
internal data structures as well.)

Long term we should have the server just send us ids here,
since we are now hydrating info from stream data in all places.

static/js/ui_init.js

@@ -410,7 +410,8 @@ exports.initialize_everything = function () {
     const stream_data_params = pop_fields(
         'subscriptions',
         'unsubscribed',
-        'never_subscribed'
+        'never_subscribed',
+        'realm_default_streams'
     );
 
     const user_groups_params = pop_fields(