Group Member Admin tweaks:

 * Show Group filter before User filter, since it's more commonly useful and also
   has fewer entries, so it's easier to scroll past if you want the other

 * Allow text search on user email, user name, and group slug
add extra user-related fields to GroupMember CSV export
allow export_action to traverse model relations if the specified fields include dunders, and allow the ModelAdmin object to specify a custom csv_export_fields list if it wants to be different than list_display