Simplify code + sheet data gathering

 * handle single vs multiple records properly in get_sheet_data()
 * _get_record_extras() is static
 * don't add record extras when calling load() or get_all()
 * extras for skills (for checkboxes)
 * automatic addition of ID into sheet data keys as necessary (for single records only)
 * simpler display of skills info for characterSheet
 * simpler updating of weapon records