message: Feature/#586: Check if course exists
  Moved regex to decorator.
diff: @@ -217,14 +217,8 @@ export class CourseController {
  * @apiError NotFoundError Includes implicit authorization check.
  * @apiError ForbiddenError (Redundant) Authorization check.
  */
  - @Get('/:id')
  + @Get('/:id([a-fA-F0-9]{24})')
  async getCourse(@Param('id') id: string, @CurrentUser() currentUser: IUser) {
  - const regex = /[g-z]/gi; // Check if id only contains characters from a-f
  -
  - if (id.search(regex) !== -1 || id.length !== 24) {
  - throw new NotFoundError();
  - }
  -
  const course = await Course.findOne({
  ...this.userReadConditions(currentUser),
  _id: id
  