Refactor handling of environment variables (#2303)

* Refactor environment variables

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

src/hooks/common/use-base-url.tsx

@@ -0,0 +1,33 @@
+/*
+ * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { useContext, useMemo } from 'react'
+import { useRouter } from 'next/router'
+import { baseUrlContext } from '../../components/common/base-url/base-url-context-provider'
+
+export enum ORIGIN {
+  EDITOR,
+  RENDERER,
+  CURRENT_PAGE
+}
+
+/**
+ * Provides the base urls for the editor and the renderer.
+ */
+export const useBaseUrl = (origin = ORIGIN.CURRENT_PAGE): string => {
+  const baseUrls = useContext(baseUrlContext)
+  if (!baseUrls) {
+    throw new Error('No base url context received. Did you forget to use the provider component?')
+  }
+
+  const router = useRouter()
+
+  return useMemo(() => {
+    return (router.route === '/render' && origin === ORIGIN.CURRENT_PAGE) || origin === ORIGIN.RENDERER
+      ? baseUrls.renderer
+      : baseUrls.editor
+  }, [origin, baseUrls.renderer, baseUrls.editor, router.route])
+}