Non-accessible menu entries are hidden from the admin UI

Problem/Motivation

When creating a link to a route that is not accessible to current user, (typically /user/login), the entry is not displayed in the administration user interface.

This issue feels very similar to an old one on the core: https://www.drupal.org/project/drupal/issues/2492513

Steps to reproduce

Create a menu link targeting /user/login.

Proposed resolution

Use menu_ui.menu_tree_manipulators:checkAccess (instead of menu.default_tree_manipulators:checkAccess) in \Drupal\bigmenu\BigMenuForm::getTree, to mimic what Drupal's core does in \Drupal\menu_ui\MenuForm::buildOverviewForm.

Remaining tasks

Fix source code.

User interface changes

None.

API changes

None.

Data model changes

None.