Allow enter key to submit an alert word.

This fixes a mild regression in 6bd2a0315ff56a20027074d65ccaa094bd35e63f,
which fixed a bigger bug.  That change added the event parameter to
add_alert_word(); this change updates one of the callers to now
supply it.

(imported from commit c6375abb8bd22d12c538cd7439462aae4665ee38)

static/js/alert_words_ui.js

@@ -61,7 +61,7 @@ $(function () {
             event.preventDefault();
 
             var word = $(event.target).val();
-            add_alert_word(word);
+            add_alert_word(word, event);
         }
     });
 });