Add audio/video link feedback in batch.js

[nickatomlin]

batch.js currently gives feedback if audio/video files are detected as missing, but this may not be correct. For example, the 20170807_ideas_for_documentation_ml.eaf file does not specify a paired video, even though one exists.

Therefore, the file does not pair with the video and no warning is shown. Something like "Media: 🎧" in the console would suggest that no video was paired, which might be helpful.