Set default sponsor
Set default sponsor

Remove the need to have an entry for every year by creating a default
value. This tripped me up when trying to set up 2021. No reason we
should have to keep adding to this list, especially since Go Congresses
haven't had any official sponsors since 2012, apparently.

usgo/gocongress,usgo/gocongress,usgo/gocongress,usgo/gocongress