Add DOWNLOAD button get get mp3

[colinpollock]

Whenever the user updates the pronunciation, the JS calls the Flask API (which calls Polly, encodes the audio as a base64 string, and passes it back to the JS). The JS creates an Audio and plays it.

It'd be nice to also have a button to download the audio. It may be possible to do this by having the user save the Audio directly. Or I may need to have the server store the audio file in S3 and pass that link back to the JS and show it to the user.