Train a digit classifier from the GUI

Currently, we have some command-line tools for loading a trained digit classifier model and performing IDing-related tasks like extracting digits from a page image, assigning IDs from a classlist to model predictions, etc. This branch adds a simple way to train a digit classifier model and run predictions on test-papers from the manager browser GUI.

In order to not keep this branch open ad infinitum, I've made running predictions from the GUI a new task separate from this MR.