Schedule nightly build of model

[jo-elimu]

Since the training datasets change (and improve) over time, we want to automate the process of building the machine learning model.

Configure a GitHub workflow scheduled to run once per night, which does the following:

• Download the newest version of the training datasets, and commit the changes (if any)
• Train the model, and commit the changes in prediction scores (if any)
• Assign a version number to the saved model, and prepare a release so that other applications can import and use the updated model.