Issue #1814916 by andypost, tim.plunkett, sun, yoroy: Convert menus into entities.

core/includes/install.inc

core/includes/menu.inc

core/modules/block/block.module

core/modules/field_ui/field_ui.module

core/modules/menu/config/menu.menu.account.yml

core/modules/menu/config/menu.menu.admin.yml

core/modules/menu/config/menu.menu.footer.yml

core/modules/menu/config/menu.menu.main.yml

core/modules/menu/config/menu.menu.tools.yml

core/modules/menu/lib/Drupal/menu/MenuFormController.php

core/modules/menu/lib/Drupal/menu/MenuListController.php

core/modules/menu/lib/Drupal/menu/Tests/MenuTest.php

core/modules/menu/menu.admin.css

core/modules/menu/menu.admin.inc

core/modules/menu/menu.api.php

core/modules/menu/menu.install

core/modules/menu/menu.module

...ules/system/lib/Drupal/system/Plugin/Core/Entity/Menu.php