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/hooks/common/use-array-string-url-parameter.ts

@@ -0,0 +1,23 @@
+/*
+ * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { useRouter } from 'next/router'
+import { useMemo } from 'react'
+
+/**
+ * Extracts the parameter from the router expected to be an array of values.
+ *
+ * @param parameter The parameter to extract
+ * @return An array of values extracted from the router.
+ */
+export const useArrayStringUrlParameter = (parameter: string): string[] => {
+  const router = useRouter()
+
+  return useMemo(() => {
+    const value = router.query[parameter]
+    return (typeof value === 'string' ? [value] : value) ?? []
+  }, [parameter, router.query])
+}