Adding sign in scenarios for FXA

This is for issue https://github.com/mozilla/marketplace-tests/issues/553
@krupa requested more scenarios to login for FXA. This will cover it for Desktop
You mean create another page object for the My Apps page? Yes, I guess we should if it is a page that we are navigating to and then verifying. Also, if we are getting to the My Apps page from the settings page, then we should click the link, not go to it directly via `selenium.get()`