Added a wait for the topics to be visible.

test_that_article_can_be_edited is failing on Jenkins because the test tries to select a topic even though they are not visible. This might be caused by the fact that the list did not expend fast enough.

http://selenium.qa.mtv2.mozilla.com:8080/view/All%20not%20B2G/job/sumo.stage/1358/HTML_Report/

Also change a wait from present to visible in open_description_form. The elements are always present, they are just hidden.