chore: Move to Typescript (#2783)

This PR moves the entire project to Typescript. Due to the ~1000 ignores this will lead to a messy codebase for a while, but the churn is worth it – all of those ignore comments are places that were never type-safe previously.

closes #1282

server/__mocks__/events.js

@@ -1,4 +0,0 @@
-// @flow
-export default {
-  add: () => {},
-};

server/__mocks__/events.ts

@@ -0,0 +1,5 @@
+export default {
+  add: () => {
+    // empty
+  },
+};