Add a wait for the topics to be displayed before clicking

Fixes #229

This should address the intermittent failure on CI. E.g. [1]

[1] http://qa-selenium.mv.mozilla.com:8080/view/SUMO/job/sumo.stage/243/testReport/junit/tests.desktop.test_login_logout/TestLoginLogout/test_logout_from_edit_kb_article_page/

Btw, this one was a bitch to fix. http://www.youtube.com/watch?v=DCrDuGyZ6FA (NSFW)