Text audience CSV headers

For school administrators using GiveCampus Outreach. Controls can differ between the classic email editor and the newer Composer editor.

The ordinary text-recipient upload requires the exact headers external_id, first_name, last_name and phone_number. Header validation trims surrounding whitespace but does not make arbitrary spellings/case equivalent. Keep the required headers even when a field's row value is blank; header validation and recipient eligibility are different checks. An exclusion upload is separate and requires only phone_number in the path.