CSA - move config back into templates, out of app-scripts-* packages

[FredKSchott]

/cc @drwpow

Background

• We created CSA apps following a model similar to create-react-app, where config was moved into a lower package so that it could be managed and upgraded by us, and not the final user.
• We've come to learn that this was a misunderstanding of what a "starter app" would look like in Snowpack vs. Webpack. Because Snowpack is so much easier to configure, our goal isn't to hide away 100s of lines of bundler configuration complexity from you. Instead, our goal is to show you a simple config example and give you clear guidance on how to extend that configuration.

Changes

• Remove references to app-scripts for everything except testing (given that we hope to move off of Jest soon, spending time migrating this config seems like a waste of effort).
• Move app-scripts config into the templates themselves (mainly babel, tsconfig, and snowpack config files)
• Add/clean up docs
• Clean up some features missing across templates

Testing

• Covered by existing tests.

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[drwpow]

LGTM, but should the app-scripts-* directories just be deleted?