Allow for scrolling of stream membership table again.

This adds perfectScrollbar to the `.subscriber_list_container` to
allow for the table to scroll naturally again. This was broken
because when perfectScrollbar is put on the parent element, any
naturally scrolling element within it will not scroll naturally
anymore.

Tweaked by tabbott to update the scrollbar on rerender.

Fixes: #6215.

static/js/stream_edit.js

@@ -33,6 +33,7 @@ function rerender_subscribers_list(sub) {
     if (subscribers_list) {
         subscribers_list.data(emails);
         subscribers_list.render();
+        ui.update_scrollbar($(".subscriber_list_container"));
     }
 }
 
@@ -197,6 +198,8 @@ function show_subscription_settings(sub_row) {
         },
     }).init();
 
+    ui.set_up_scrollbar($(".subscriber_list_container"));
+
     sub_settings.find('input[name="principal"]').typeahead({
         source: people.get_realm_persons, // This is a function.
         items: 5,

static/styles/subscriptions.css

@@ -291,6 +291,7 @@ form#add_new_subscription {
 }
 
 .subscriber_list_container {
+    position: relative;
     max-height: 300px;
     overflow: auto;
     text-align: left;