fix: Move content into to frontend directory

Doing this BEFORE the merge prevents a lot of merge conflicts.

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

frontend/src/pages/_app.tsx

@@ -0,0 +1,60 @@
+/*
+ * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+import type { AppInitialProps, AppProps } from 'next/app'
+import { ErrorBoundary } from '../components/error-boundary/error-boundary'
+import { ApplicationLoader } from '../components/application-loader/application-loader'
+import '../../global-styles/dark.scss'
+import '../../global-styles/index.scss'
+import { BaseHead } from '../components/layout/base-head'
+import { StoreProvider } from '../redux/store-provider'
+import { UiNotificationBoundary } from '../components/notifications/ui-notification-boundary'
+import { ExpectedOriginBoundary } from '../utils/uri-origin-boundary'
+import React from 'react'
+import { BaseUrlContextProvider } from '../components/common/base-url/base-url-context-provider'
+import type { BaseUrls } from '../components/common/base-url/base-url-context-provider'
+import { BaseUrlFromEnvExtractor } from '../utils/base-url-from-env-extractor'
+
+interface AppPageProps {
+  baseUrls: BaseUrls | undefined
+}
+
+/**
+ * The actual hedgedoc next js app.
+ * Provides necessary wrapper components to every page.
+ */
+function HedgeDocApp({ Component, pageProps }: AppProps<AppPageProps>) {
+  return (
+    <BaseUrlContextProvider baseUrls={pageProps.baseUrls}>
+      <StoreProvider>
+        <ExpectedOriginBoundary>
+          <BaseHead />
+          <ApplicationLoader>
+            <ErrorBoundary>
+              <UiNotificationBoundary>
+                <Component {...pageProps} />
+              </UiNotificationBoundary>
+            </ErrorBoundary>
+          </ApplicationLoader>
+        </ExpectedOriginBoundary>
+      </StoreProvider>
+    </BaseUrlContextProvider>
+  )
+}
+
+const baseUrlFromEnvExtractor: BaseUrlFromEnvExtractor = new BaseUrlFromEnvExtractor()
+
+HedgeDocApp.getInitialProps = (): AppInitialProps<AppPageProps> => {
+  const baseUrls = baseUrlFromEnvExtractor.extractBaseUrls().orElse(undefined)
+
+  return {
+    pageProps: {
+      baseUrls
+    }
+  }
+}
+
+// noinspection JSUnusedGlobalSymbols
+export default HedgeDocApp