Convert to Typescript

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At some point it would likely be in our best interests to convert from Flow to Typescript, which has a much more active community around it.

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Steps to set up:

1. Install dependencies npm install --save typescript @types/node @types/react @types/react-dom @types/jest
2. Replace jsconfig.json with tsconfig.json
3. Update Dockerfiles to include tsconfig.json instead of jsconfig.json
4. Check Jest coverage collection and styleguide config to make sure they include .ts and .tsx files

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This process has begun -- any new files should be written in TypeScript.