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/markdown-renderer/extensions/image/communicator-image-lightbox.tsx

@@ -0,0 +1,40 @@
+/*
+ * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import type { ImageDetails } from '../../../render-page/window-post-message-communicator/rendering-message'
+import React, { useCallback, useState } from 'react'
+import { useBooleanState } from '../../../../hooks/common/use-boolean-state'
+import { ImageLightboxModal } from './image-lightbox-modal'
+import { useExtensionEventEmitterHandler } from '../../hooks/use-extension-event-emitter'
+import { SHOW_IMAGE_LIGHTBOX_EVENT_NAME } from './event-emitting-proxy-image-frame'
+
+/**
+ * Handles messages from the render in the iframe to open a {@link ImageLightboxModal}.
+ */
+export const CommunicatorImageLightbox: React.FC = () => {
+  const [lightboxDetails, setLightboxDetails] = useState<ImageDetails | undefined>(undefined)
+  const [modalVisibility, showModal, closeModal] = useBooleanState()
+
+  const handler = useCallback(
+    (values: ImageDetails) => {
+      setLightboxDetails?.(values)
+      showModal()
+    },
+    [showModal]
+  )
+
+  useExtensionEventEmitterHandler(SHOW_IMAGE_LIGHTBOX_EVENT_NAME, handler)
+
+  return (
+    <ImageLightboxModal
+      show={modalVisibility}
+      onHide={closeModal}
+      src={lightboxDetails?.src}
+      alt={lightboxDetails?.alt}
+      title={lightboxDetails?.title}
+    />
+  )
+}