Music: Fix level_data loading when using Next button

[breville]

This fixes an issue in which pressing the "Next" button inside Music Lab was loading both the current level and the next level's "level_data" more or less simultaneously, and sometimes the new level would show the previous level's instructions.

@molly-moen did an excellent analysis which is reproduced here with slight tweaks:

When you click "Next" in a script-level, the following things happen:

1. onNextPanel calls ProgressManager’s next, which increments the step in the ProgressManager and then calls back to MusicView’s onProgressChange.
2. onProgressChange sets the current progress state in musicRedux, which is different from setting the current level in progressRedux.
3. onNextPanel then calls handlePanelChange, which fires off an async method to load the next level. It calls the update with currentLevelIndex, which has not been changed yet.
4. Back in onNextPanel, if we are a script-level, then we dispatch a navigateToLevelId in progressRedux. When we are a script-level, this will result in the new level index arriving in the currentLevelIndex prop.
5. In componentDidUpdate, we see that currentLevelIndex has changed, so we call handlePanelChange again with the correct new level index, which fires off a second async method to load the new level.
6. In some repro situations (sometimes on the first load for a given user on the progression), the first request finishes after the second request, so we see the old level data.

Really, we are prematurely calling handlePanelChange in onNextPanel.

Given this analysis, the fix indeed seems to be to simply remove the unnecessary call to handlePanelChange in onNextPanel.

[sanchitmalhotra126]

🎉 thanks Brendan for the quick fix and thanks Molly for the diagnosis! I'm going to go ahead and merge this now to get this in for today's deploy.