Add option to download individual clips and spectrogram photos

[RichardLitt]

Often, I don't actually need all of the clips - but a single clip, which I want to upload to eBird. Being able to download the clip directly from the Clip album would be excellent.

Related to #167 in that it would be ideal if I could add two seconds of background noise on either side, as this would be for public record on Macaulay Library.

[RichardLitt]

I tried to implement this by copying the playButton functions in clip-view.js, but ran into an issue where I wanted to add in an npm library to convert the audiobuffer into a wav so that I didn't have to do that work myself. However, it looks like there is no current loading of any JS packages anywhere. Is this entirely vanilla JavaScript? Perhaps there's another function I could use, which is used in the Export clips job.