settings: Call maybe_disable_widgets() in set_up() methods.

We now rely on set_up() methods to call their
own module-specific versions of maybe_disable_widgets()
in the codepath for admin_sections.load_admin_section().

And then for live updates, we just explicitly call
all four modules that support maybe_disable_widgets().

This should make switching between sections slightly faster,
and it also reduces the risk of module A messing with
module B's state.  (Granted, we have lots of other ways
that modules can mess with each other's state.)

static/js/settings_streams.js

@@ -109,6 +109,11 @@ exports.delete_default_stream = function (stream_name, default_stream_row, alert
 };
 
 exports.set_up = function () {
+    exports.build_page();
+    exports.maybe_disable_widgets();
+};
+
+exports.build_page = function () {
     meta.loaded = true;
 
     exports.update_default_streams_table();