#27/automate toolbox creation

[sansaid]

Created a generate-exercises.js which creates the src/Exercises folder that currently exists. However, one difference to the old structure of src/Exercises is that it creates an index.js in the root of src/Exercises which is exported to App.js so we don't have to maintain a manual list any more.

generate-exercises.js takes the inputs in the ./exercises folder, looks for references to block names enclosed in <!--block--> tags in the exercise .md and then locates a matching block name in the blocks-to-categories.json in order to generate the toolbox for that exercise. blocks-to-categories.json should be maintained as a list of identifiable blocks ID'd by human readable names which teachers can use inside their exercise .md.

The package.json has also been updated so that the exercises are automatically generated whenever anyone runs npm run start/build/test.

What's not included in this PR is a README of how to add exercises and how this process works. If approval is granted and we're happy with this approach, I'll add a commit to create a README with instructions.

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View rendered client/exercises/01-stuff.md
View rendered client/exercises/02-more-stuff.md
View rendered client/exercises/03-test-content.md
View rendered client/src/Exercises/01-stuff/lesson.md
View rendered client/src/Exercises/02-more-stuff/lesson.md

[gregdyke]

I'm not massively a fan of the tags, because I think the mds are already pretty consistently xxxxxx block, which is marginally easier to read and write? (also someone would need to go over all the mds and change them, which isn't the end of the world, but still effort)