Open the composebox on sidebar subject selection.

This was requested by CUSTOMER28 and would also bring symmetry with
the user sidebar, which already does this.

(imported from commit 4dca3c957d3a710f00bbb34a7cd7dfe6074da8f7)

static/js/ui.js

@@ -1282,6 +1282,9 @@ $(function () {
                          ['topic', subject]],
                         {select_first_unread: true, trigger: 'sidebar'});
 
+        compose.start('stream', {stream: stream, subject: subject,
+                                 trigger: 'sidebar subject click'});
+
         e.preventDefault();
     });