WIP: Enable strict-mode for TypeScript #1783

Basically going with the non-null assertion operator in almost all places, because as mentioned in https://github.com/sindresorhus/refined-github/issues/1735#issuecomment-462027092, the assumption with the extension is that the DOM nodes referenced in most cases are expected to be there. If they aren't, the extension breaks, a bug is filed and it's fixed.

One thing to note @bfred-it, is currently dom-chef types need to handle JSX (Element and IntrinsicElements) or we import @types/react.

Closes https://github.com/sindresorhus/refined-github/issues/1735