Install and use ToastUi markdown editor

Replace all usages of pagedown-bootstrap editor with the new editor.
Add styles to ensure the editor preview matches the final output.

app/javascript/toast-ui.js

@@ -0,0 +1,7 @@
+/* eslint no-console:0 */
+
+// JS
+import ToastUi from "@toast-ui/editor";
+// Import German locales (english ones are included by default)
+import "@toast-ui/editor/dist/i18n/de-de";
+window.ToastUi = ToastUi