ML: Improve the machine learning pipeline

[jo-elimu]

Streamline the process of building, training, evaluating and deploying our machine learning models.

Storybook Reading Level Predictor

• [x] https://github.com/elimu-ai/ml-storybook-reading-level/issues/11
• [x] https://github.com/elimu-ai/ml-storybook-reading-level/issues/8
• [x] https://github.com/elimu-ai/ml-storybook-reading-level/issues/9
• [x] Add GitHub workflow for saving model as PMML
• [x] https://github.com/elimu-ai/ml-storybook-reading-level/issues/12
• [x] https://github.com/elimu-ai/ml-storybook-reading-level/issues/20
• [ ] https://github.com/elimu-ai/ml-storybook-reading-level/issues/26
• [ ] https://github.com/elimu-ai/ml-storybook-reading-level/issues/27
• [ ] https://github.com/elimu-ai/webapp/issues/1920