Improve Code Quality in attendance.controller.js

[aeksco]

Clean up tasks:

• Replace all var statements with either let or const.

• Replace per-file definitions of handleError with the one defined in api/lib/helpers.js:

import { handleError } from '../lib/helpers'

• Make whitespace consistent in file (i.e. if(err){ ... should be if (err) { ...)

• Replace " (double quote) with ' (single quotes) for string constants

• Replace function keyword with => function syntax, i.e.: this:

  ClassYear.findOne({"dayCodes.code":code})
  .exec(function(err, classYear){
    ...
  })

  becomes:

  ClassYear.findOne({"dayCodes.code":code})
  .exec((err, classYear) => {
    ...
  })

• Replace Mongoose callbacks with promises:

this:

  ClassYear.find({}, function (err, model) {
    if (err) return handleError(err)
    ...
  })

becomes:

  ClassYear.find({})
  .then((model) => {
    ...
  })
  .catch((err) => {
    handleError(err)
  })