move shirt size constants to `Shirt` model
remove deprecated `primary_attendee` factory
remove display of primary attendee from user summary
show email instead of primary atnd. name (deprecated)
extract `populate_atrs_for_new_form` into `Attendee`

to enable better testing
copy country and phone from first created, not primary (deprecated)
let them make their own attendee record
remove deprecated `primary_attendee`
partition invoices by email, not prim. atnd. name (deprecated)
replace deprecated primary_attendee with email
finish removing user.primary_attendee (deprecated)
drop column `attendees.is_primary`
defunct notes
display shirt size code instead of name
extract `birthdate_in_congress_year`
remove defunct grammar methods
move `Attendee.internal_attributes` to `AttendeesCsvExporter::AttendeeAttributes::INTERNAL_ATRS`