System: Add option to disable netboot

[SchoolGuy]

Is your feature request related to a problem? Please describe.

As a user
I want to be able to disable network booting
so that my systems don't end up in a bootloop.

Describe the solution you'd like

Add an action button for systems that allow disabling network booting for a given system.

Describe alternatives you've considered

None.

Additional context

This should not be done via a modify_system & save_system call but instead the method disable_netboot should be used as this is also used by systems directly.

Furthermore, this button must be invisible when the setting pxe_just_once is False.