message: Add comments for export api
diff: @@ -9,18 +9,81 @@ import {Unit} from '../models/units/Unit';
  @Authorized(['teacher', 'admin'])
  export class ExportController {
  + /**
  + * @api {get} /api/export/course/:id Export course
  + * @apiName GetExportCourse
  + * @apiGroup Export
  + * @apiPermission teacher
  + * @apiPermission admin
  + *
  + * @apiParam {String} id Course ID.
  + *
  + * @apiSuccess {Course} course Course for export.
  + *
  + * @apiSuccessExample {json} Success-Response:
  + * {
  + * "name": "Test 101",
  + * "description": "Some course desc",
  + * "enrollType": "whitelist",
  + * "lectures": [{
  + * "name": "Lecture One",
  + * "description": "Some lecture desc",
  + * "units": []
  + * }],
  + * "hasAccessKey": false
  + * }
  + */
  @Get('/course/:id')
  async exportCourse(@Param('id') id: string) {
  const course = await Course.findById(id);
  return course.exportJSON();
  }
  + /**
  + * @api {get} /api/export/lecture/:id Export lecture
  + * @apiName GetExportLecture
  + * @apiGroup Export
  + * @apiPermission teacher
  + * @apiPermission admin
  + *
  + * @apiParam {String} id Lecture ID.
  + *
  + * @apiSuccess {Lecture} lecture Lecture for export.
  + *
  + * @apiSuccessExample {json} Success-Response:
  + * {
  + * "name": "Lecture One",
  + * "description": "Some lecture desc",
  + * "units": []
  + * }
  + */
  @Get('/lecture/:id')
  async exportLecture(@Param('id') id: string) {
  const lecture = await Lecture.findById(id);
  return lecture.exportJSON();
  }
  + /**
  + * @api {get} /api/export/unit/:id Export unit
  + * @apiName GetExportUnit
  + * @apiGroup Export
  + * @apiPermission teacher
  + * @apiPermission admin
  + *
  + * @apiParam {String} id Unit ID.
  + *
  + * @apiSuccess {Unit} unit Unit for export.
  + *
  + * @apiSuccessExample {json} Success-Response:
  + * {
  + * "progressable": false,
  + * "weight": 0,
  + * "name": "First unit",
  + * "description": null,
  + * "markdown": "Welcome, this is the start",
  + * "__t": "free-text"
  + * }
  + */
  @Get('/unit/:id')
  async exportUnit(@Param('id') id: string) {
  const unit = await Unit.findById(id);
  