Fix speaker admin page filters to do an AND query.
Fix speaker admin page filters to do an AND query.

Otherwise, the Django filters work in OR more, giving speakers
with at least on accepted talk in all years, which also have submitted
a talk for the selected conference year.

Remove the default filter settings again, since these don't allow
to select all speakers in the database.

malemburg/epcon,malemburg/epcon,malemburg/epcon,artcz/epcon,matrixise/epcon,EuroPython/epcon,EuroPython/epcon,matrixise/epcon,matrixise/epcon,artcz/epcon,artcz/epcon,matrixise/epcon,malemburg/epcon,matrixise/epcon,EuroPython/epcon,malemburg/epcon,EuroPython/epcon,artcz/epcon,artcz/epcon,malemburg/epcon,matrixise/epcon,artcz/epcon