Added a wait for ajax to be ready when watching/unwatching a page.

[test_visitor_can_watch_page][link] is failing on Jenkins because the message will reappear when clicking on the main page link.

[link]: http://selenium.qa.mtv2.mozilla.com:8080/view/All%20not%20B2G/job/wiki.prod/821/BROWSER_NAME=firefox,PLATFORM=windows/testReport/tests.test_watch_main_page_after_log_in/TestWatchPage/test_visitor_can_watch_page/