Address issue where the "Remember Me" screen does not appear during the returning user login flow

The issue is that, when a user logs back into Persona, sometimes the "Remember Me" screen appears (if more than 60 seconds has elapsed) and sometimes it doesn't. We have a test in Moztrap that logs out and back in and the screen was not appearing, which caused the existing bidpom code to hang on a `WebDriverWait`.

The changes to `sign_in.py` address that issue directly.

This pull request also adds a helper method to bidpom called `sign_in_returning_user` which does not require the calling application to know anything about the state of the user or application (i.e., there is no requirement for an `expect` argument). Bidpom itself is able to address the different flows.

I admit that the code is a bit hacky. If anyone has suggestions for improvements they are welcome. The challenge is that we BID window just disappears during certain flows.

Note that I also updated the baseurl in `mozwebqa.cfg` to reflect the address we use in CI.