function getSubFolder_(objParentFolder, subFolderName) {

  // Iterates subfolders of parent folder to check if folder already exists.
  const subFolders = objParentFolder.getFolders();
  while (subFolders.hasNext()) {
    let folder = subFolders.next();

    // Returns the existing folder if found.
    if (folder.getName() === subFolderName) {
      return folder;
    }
  }
  // Creates a new folder if one doesn't already exist.
  return objParentFolder.createFolder(subFolderName)
    .setDescription(`Created by ${APP_TITLE} application to store uploaded Forms files.`);
}
Returns a Drive folder under the passed in objParentFolder parent
folder. Checks if folder of same name exists before creating, returning 
the existing folder or the newly created one if not found.

@param {object} objParentFolder - Drive folder as an object.
@param {string} subFolderName - Name of subfolder to create/return.
@return {object} Drive folder