Raise MAX_CONTENT_TYPE_BYTES to 80, lower MAX_PRIVATE_MESSAGE_TEXT_LENGTH

This branch raises MAX_CONTENT_TYPE_BYTES to 80 in case we ever want to send "application/vnd.openxmlformats-officedocument.wordprocessingml.document" attachments.

MAX_PRIVATE_MESSAGE_TEXT_LENGTH is lowered to make room for longer content types in the list of attachments. The validator will continue to accept messages with the old length (now called MAX_PRIVATE_MESSAGE_INCOMING_TEXT_LENGTH) for "a reasonable migration period", to allow time for contacts to upgrade to the new format.