Convert to TypeScript

[nickserv]

Since this is a Gatsby site with some interactive pages like Contact and Jobs, there's a bit of logic. It can be tricky to refactor this without tests, though we don't really need to test particularly complex use cases, so I think TypeScript would work pretty well to give us a better promise of stability.

Large TypeScript migrations can be complex, though Airbnb has a new cool called ts-migrate which could make it easier than the traditional file-by-file migration.