Implement orderBy parameter on events query

[mattlindsey]

Allow frontend to request events ordered ASC or DESC.
So the following should work at http://localhost:3000/graphiql:

query { 
    events(orderBy: {field: startDatetime, direction: ASC}) {
      id
      category
      duration
      name
      repeats
      startDatetime
      timeZone
    }
}

See https://stackoverflow.com/questions/49458718/trouble-with-graphql-orderby-query