Add fill_form_and_submit

The pattern of filling a form then immediately submitting seems to be the most common interaction, and the more I use this the more I think they should be combined.

I'd like to keep both helpers separate as well in case there is something that needs to be done between the interactions, so this would be an addition to the public interface.
Done in d95ba7a83a751753f96c49b19ba33a595bc85b0a