Add nightly job for importing the latest version of the storybook reading level model

[jo-elimu]

Description

Add a GitHub workflow for copying https://github.com/elimu-ai/ml-storybook-reading-level/blob/main/pmml/step2_train/step2_2_model.pmml to https://github.com/elimu-ai/webapp/blob/main/src/main/resources/ai/elimu/web/content/storybook/step2_2_model.pmml

Why?

This is needed because we don't want to manually copy the model file into the webapp every time the reading level model changes.

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• Relates to Use machine learning model to predict storybook reading level #1821
• Relates to Move machine learning code into utils class #1825