Fix broken tests by clicking on the Topic header before trying to select a topic

Also adds code to attempt to dismiss the staging message on each page. That was also causing problems with these tests. The code I added isn't guaranteed to dismiss the message every time, but it should address most cases, and we may have to address others than come up on a case-by-case basis.

This fixes the 7 failures as seen at http://qa-selenium.mv.mozilla.com:8080/view/SUMO/job/sumo.dev/5828/testReport/

Adhoc run on Jenkins: http://qa-selenium.mv.mozilla.com:8080/job/sumo.stage.saucelabs.topic_fix/5/