(selenium tests) fix enemy #1 - turn two distinct transactions into a signle...

(selenium tests) fix enemy #1 - turn two distinct transactions into a signle transaction to fix StaleElementErrors seen during ChangePassword tests /cc @klrmn @zac @6a68

automation-tests/browserid/pages/account_manager.py

@@ -28,7 +28,7 @@ class AccountManager(Base):
 
     @property
     def signed_in(self):
-        return 'not_authenticated' not in self.selenium.find_element(By.TAG_NAME, 'body').get_attribute('class')
+        return not self.selenium.find_element(By.CSS_SELECTOR, 'body.not_authenticated')
 
     @property
     def emails(self):