Export plot objects as plotly Figures

[BryanBaird]

This may be a simpler one, but the current plotting code includes multiple steps:

1. Some helpful data munging
2. Some aesthetic design options for the plots
3. A call to plotly express show() the ultimately generated plot here: https://github.com/mozilla/mozaic-forecasting/blob/2becad6a9b372c8f0d732f91ee92f6b8d7b06fed/mozaic/plotting.py#L18C1-L40

By either just removing this fig.show() call, or setting up a different function call that outputs the plot object, it will still automatically render in most typical Jupyter/iPython/Notebook contexts. But it will have the benefit of being customizable after-the-fact, which the current approach is not--at least not without extensive workarounds.

Example: Wanting to manually specify the x-axis date range of the plot, or change the title/legends, or add an annotation region.