typeahead: Fix bug where change event was not triggered post selection.

This fixes a regression introduced in the commit making typeahead undo
friendly, d88ce61801.

This fixes the bug where selecting a topic via keyboard in the compose
recipient area did not update the fade.

web/src/bootstrap_typeahead.js

@@ -201,6 +201,7 @@ Typeahead.prototype = {
             // select / highlight the minimal text to be replaced
             this.$element[0].setSelectionRange(from, to_before);
             insertTextIntoField(this.$element[0], replacement);
+            this.$element.trigger("change");
         }
 
         return this.hide();