Replace i18n by a custom implementation (#638)

There are two problems with the current localization strategy:
- We download the translations on-demand, which means that it does a serial roundtrip for nothing.
- withTranslation helper actually renders the app 3 times on startup, instead of once (I haven't tried to debug it)

src/components/StoredScenesList.tsx

@@ -1,6 +1,6 @@
 import React from "react";
-import { useTranslation } from "react-i18next";
 import { PreviousScene } from "../scene/types";
+import { t } from "../i18n";
 
 interface StoredScenesListProps {
   scenes: PreviousScene[];
@@ -13,8 +13,6 @@ export function StoredScenesList({
   currentId,
   onChange,
 }: StoredScenesListProps) {
-  const { t } = useTranslation();
-
   return (
     <React.Fragment>
       <select