Make the page object wait for pages to load by default

Remove unnecessary steps from test_hide_email_checkbox_works

test_hide_email_checkbox_works was failing intermittently which may have been related to bad waits, and also seems to be related to an invalid assertion in the test. This PR addresses both of those issues.

This will impact many tests, so I'm running an adhoc here: http://selenium.qa.mtv2.mozilla.com:8080/job/amo.dev.saucelabs.adhoc/1/