Move app scaffolding to src/app

This commit executes a micro-refactor to move all top-level app setup
code out of src/plugins/artifact/editor and into src/app. The observed
behavior from `yarn start`, which is to show the artifact editor, is
unchanged.

config/paths.js

@@ -41,9 +41,9 @@ function getServedPath(appPackageJson) {
 module.exports = {
   dotenv: resolveApp(".env"),
   appBuild: resolveApp("build"),
-  appPublic: resolveApp("src/plugins/artifact/editor/public"),
+  appPublic: resolveApp("src/app/public"),
-  appHtml: resolveApp("src/plugins/artifact/editor/public/index.html"),
+  appHtml: resolveApp("src/app/public/index.html"),
-  appIndexJs: resolveApp("src/plugins/artifact/editor/index.js"),
+  appIndexJs: resolveApp("src/app/index.js"),
   appPackageJson: resolveApp("package.json"),
   appSrc: resolveApp("src"),
   yarnLockFile: resolveApp("yarn.lock"),

src/app/index.js

@@ -2,7 +2,7 @@
 import React from "react";
 import ReactDOM from "react-dom";
 import "./index.css";
-import App from "./App";
+import App from "../plugins/artifact/editor/App";
 import registerServiceWorker from "./registerServiceWorker";
 
 const root = document.getElementById("root");