Building on #275 (#289)

* Upgrade devDependcies. Replace UglifyJS ⚰ with TerserJS 👶 Fix TypeScript compiler errors

* Remove babel and associated plugins

* Re-enable strictNullChecks and noImplicitAny

* Use surma's better ga type definition.
`ts-ignore` document.activeElement potential null warnings

* Avoiding ignores

src/missing-types.d.ts

@@ -39,7 +39,3 @@ declare var ga: {
   (...args: any[]): void;
   q: any[];
 };
-
-interface Window {
-  ga: typeof ga;
-}