Add link to show all streams when you are zoomed in.

When your left sidebar is zoomed to show just one stream,
there is a link to to show all streams again.

(imported from commit 92f39b042168c443cbb9f524bf892557ef492551)

static/js/stream_list.js

@@ -443,6 +443,12 @@ $(function () {
         previous_sort_order = undefined;
     });
 
+    $('.show-all-streams').on('click', function (e) {
+        toggle_zoom();
+        e.preventDefault();
+        e.stopPropagation();
+    });
+
     $('#stream_filters').on('dblclick', 'li .subscription_block', function (e) {
         if (!feature_flags.topic_zooming) {
             return;