Compute shap values for all rows at once in explain_prediction and explain_predictions_best_worst

As noted in #1821, we can compute the shap values for all the rows we want to explain at once, rather than iterating through the rows.

Here is a section of the shap documentation showing how to do [this](https://shap.readthedocs.io/en/latest/example_notebooks/tabular_examples/model_agnostic/Diabetes%20regression.html#Explain-all-the-predictions-in-the-test-set).

Marking this as blocked by #1821 because we can only do this if we compute the features for all of the rows we want to explain at once.