Fix for nova >=4.28.

[Senexis]

This pull request resolves two issues (including #189):

• Both loading buttons and danger buttons are no longer present in Laravel Nova, instead buttons are being moved to the laravel-nova-ui package. I've copied the buttons from the current Laravel Nova views.
• Resolves a potential issue when duplicating menu items. Due to the use of the ->toArray() call, appended attributes (enabledClass etc) and relational attributes (children etc) were being included, causing an unknown column issue when trying to insert the data into the database. Using a ->getAttributes() call resolves this issue while not breaking any existing functionality such as copying with children or as child.

I've not tested for backwards compatibility. This may need a Laravel Nova dependency minimum version bump, though in theory as this doesn't depend on Laravel Nova for the buttons anymore it may also be fine.

[Tarpsvo]

Awesome, thanks a ton for the PR! Everything seems solid and it's exactly how I've upgraded other packages as well since the laravel-nova-ui change in Nova. The release went out tagged at 7.1.5. Thanks again and good luck! :)