Fix failing test_edit_profile_change_display_name that expected a specif.ic username at the start of the test

Also sped up the test by adding a is_element_not_visible method to page.py

Note that test_edit_profile_change_display_name has been failing on staging [1] due to the default username for the user being changed (likely as a result of this test failing before). We could "fix" this by just updating the username for the user on staging, but it's a bad idea to have a test that depends on data being in a specific state before the test is run (data that we are not controlling via the test), so I think this fix should be implemented.

[1] http://selenium.qa.mtv2.mozilla.com:8080/job/affiliates.stage.saucelabs/1163/HTML_Report/