use Babel instead of ts-jest to speed up and simplify tests

Created by: sqs

Removed typechecking from jest tests (by removing ts-jest and just using Babel's TypeScript transpilation, per https://jestjs.io/docs/en/getting-started#using-typescript). This makes tests run faster (~2s vs. ~9s for a single test after making a change to a .ts file) and simplifies their build configuration (by removing an unnecessary tool, ts-jest, from our toolchain). You should instead rely on your editor (and CI) for typechecking.

Babel is a good choice because it is the standard for transpilation, and tslint is being deprecated in favor of Babel + eslint (so we need to switch soon anyway to get the latest lint rules, such as those for React hooks).