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
Again this method name is not appropriate. Also, I think this could be refactored out of this object all together. This logic belongs in `base.py`. I think you can make changes to `base.py` that include the current `login` method that will allow you to remove this method entirely. You can probably also eliminate duplicate code from `login_with_user_from_other_pages` at https://github.com/mozilla/marketplace-tests/pull/554/files#diff-206aaea2a9f548ea47c8627c88184445R44 by refactoring all of this.