Chore/convert to typescript (#60)

* Convert to using TypeScript

Next.js v.9 allows converting an existing project to TS by simply renaming the files as TS files

* Fix type errors: Type 'string' is not assignable to type 'number'.

* Add mention about missing typings for javascript-time-ago

* Add GA_INITIALIZED to window

* Fix type error in feed transformers

* Model MembershipInfoForm state and props with TS

* Silence the typescript warning in MembershipInfoForm

* Add threshold to Fader props

* Fix Warning: Each child in a list should have a unique "key" prop.

Sponsors don't have ids – name is probably unique and can be used as a key

* Allow key as prop for SponsorLink

* Add type of the props for SponsorLink

components/Loader.tsx

@@ -0,0 +1,12 @@
+import React from "react";
+
+export default function Loader() {
+  return (
+    <div className="sk-folding-cube">
+      <div className="sk-cube1 sk-cube" />
+      <div className="sk-cube2 sk-cube" />
+      <div className="sk-cube4 sk-cube" />
+      <div className="sk-cube3 sk-cube" />
+    </div>
+  );
+}