Move spectrogram generation to clientside javascript

[mmcauliffe]

Currently spectrogram generation is done server side once per utterance, which has the following effects:

• Time resolution on zoom is poor (1000 time steps per utterance)
• Implementation of any spectrogram settings changes would require a server request
• Potential bottleneck for many users looking at items on a single server

There exist libraries for FFT in javascript that could be used to calculate spectrograms clientside on the fly as needed, leading to better time resolution and more responsive browsing.