Dienst export does not remove "Non-members" when "Member" is checked

[bramvankooten]

When exporting emails for an education mailing, someone who is not a member anymore in Dienst was also in the export. The export was created by checking the 'Leden' checkbox, and setting the 'Onderwijs Mailing' option as yes.

[praseodym]

This is likely related to #72; the export functionality doesn't use the same definition of 'Member' as other parts of Dienst2.