Wire up frontend to use nbb export and html

- Add tests for export
- Refactor export to be more readable
- Add error notification for export that user can see
- Remove unused :ui/show-recent? state
- Cleanup and move escape fns to proper locations

src/main/frontend/state.cljs

@@ -92,7 +92,6 @@
      :ui/sidebar-collapsed-blocks           {}
      :ui/root-component                     nil
      :ui/file-component                     nil
-     :ui/show-recent?                       false
      :ui/developer-mode?                    (or (= (storage/get "developer-mode") "true")
                                                 false)
      ;; remember scroll positions of visited paths