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/components/error-pages/common-error-page.tsx

@@ -0,0 +1,47 @@
+/*
+ * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import type { PropsWithChildren } from 'react'
+import React from 'react'
+import { LandingLayout } from '../landing-layout/landing-layout'
+import { Trans, useTranslation } from 'react-i18next'
+import { ShowIf } from '../common/show-if/show-if'
+
+export interface CommonErrorPageProps {
+  titleI18nKey: string
+  descriptionI18nKey?: string
+}
+
+/**
+ * Renders a common customizable error page.
+ *
+ * @param titleI18nKey The translation key for the title of the error.
+ * @param descriptionI18nKey The translation key for the description of the error. Property is optional.
+ * @param children The optional child elements that will be displayed beneath the description.
+ */
+export const CommonErrorPage: React.FC<PropsWithChildren<CommonErrorPageProps>> = ({
+  titleI18nKey,
+  descriptionI18nKey,
+  children
+}) => {
+  useTranslation()
+
+  return (
+    <LandingLayout>
+      <div className='text-light d-flex flex-column align-items-center justify-content-center my-5'>
+        <h1>
+          <Trans i18nKey={titleI18nKey} />
+        </h1>
+        <ShowIf condition={!!descriptionI18nKey}>
+          <h3>
+            <Trans i18nKey={descriptionI18nKey} />
+          </h3>
+        </ShowIf>
+        {children}
+      </div>
+    </LandingLayout>
+  )
+}