(core) Fix browser history bug with tutorials

Summary:
History wasn't being replaced in some cases, which was
causing a bug where trying to leave a tutorial fork via the
browser's back button would navigate back to the trunk, and
trigger forking again. This effectively made it impossible to
leave a tutorial.

Also adds support for specifying custom CSS classes for
tutorial Markdown images.

Test Plan: Browser test.

Reviewers: JakubSerafin

Reviewed By: JakubSerafin

Differential Revision: https://phab.getgrist.com/D3866

app/client/models/DocPageModel.ts

@@ -240,9 +240,9 @@ export class DocPageModelImpl extends Disposable implements DocPageModel {
   public updateUrlNoReload(
     urlId: string,
     urlOpenMode: OpenDocMode,
-    options: {removeSlug?: boolean, replaceUrl?: boolean} = {removeSlug: false, replaceUrl: true}
+    options: {removeSlug?: boolean, replaceUrl?: boolean} = {}
   ) {
-    const {removeSlug, replaceUrl} = options;
+    const {removeSlug = false, replaceUrl = true} = options;
     const state = urlState().state.get();
     const nextState = {
       ...state,