Stricter tsconfig for the browser extension
Created by: lguychard
Fixes #791
Reinstates noImplicitAny
, noImplicitReturns
, strictPropertyInitialization
in the browser extension & fixes related errors.
Required some refactoring: cc46613
There is a lot more refactoring to do in phabricator/backend.tsx but I'll be addressing that in a follow-up PR.