MNIST example with bqplot image

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Make a MNIST (and fashion MNIST) example with a heatmap and a bqplot widget that shows selected digits or selected pixels as images. We will have the bqplot widget observe the selected rows/columns of the Clustergrammer2 widget. This will be a nice way of showing the linked views we can generate using widgets. We can also make the example in Jupyter Lab and position the two views next to each other.