Feature/batch upload1

This PR implements the batch upload feature with image and audio files. To upload a deck, place the images, audio files and the flashcard template in the same folder, zip the folder and upload it. In the template, the image and audio values should be the name of the files (including extensions).

This PR also includes a helper function called "helper_script_batch_upload.py" that outputs the names of the files in a given folder, separated by file types. So instead of having to type the names of files in the flashcard template, the file names can be easily copy/pasted.

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[FLASH-67](https://jira.huit.harvard.edu/browse/FLASH-67)