Missing demo code for MNIST example in Chapter 1, Activities 1 and 2

[Transigent]

This book spends a fair chunk of Chapter 1 going into depth about an example that uses the MNIST dataset, and invites the reader to train the network using the file:
train_mnist.py

But there is no such file, and there is no directory
lesson_1/exercise_a

This is a bit of an oversight as much of Chapter 1 is dedicated to walking through the tensorboard output from this code and twiddling learning rates, epoch counts etc. to get a feel for how these key components operate.