Early Track Separation

[incidentist]

Give the user the option to start separating the track early in the process, so that they can do timings, etc while the track is splitting. This requires moving the "Backing Vocals" toggle to the beginning. Put a spinner somewhere on the page to indicate that separation is happening, and change an icon somewhere to indicate that it's done.

This will also enable some features down the line:

• Having the vocal track will let us display a waveform of the vocal track during timings
• Having the instrumental track can allow the user to toggle between vocals and no-vocals during the preview.