async function create(calendar) {
  const createdCalendar = await db.Calendar.create(calendar);
  return createdCalendar.get({ plain: true });
}
@description Create a calendar.
@param {object} calendar - A calendar object.
@returns {Promise} Resolve with created calendar.
@example
gladys.calendar.create({
   name: 'Work',
   description: 'My work calendar',
   user_id: '0cd30aef-9c4e-4a23-88e3-3547971296e5',
})