Devise a better way to handle machines when Business Units or Machine Groups are deleted

[sheagcraig]

Is your feature request related to a problem? Please describe.
Current behavior is that when a Business Unit or Machine Group is deleted, the machines related through their ForeignKeys are also deletede (on_delete=models.CASCADE). This is problematic in some circumstances, as the intent may just be to restructure organizational units, but the result is the dumping of machine records.

Describe the solution you'd like
Options include

• assigning orphaned machines to a builtin default group.
• offering assign to-be-orphaned machines to another (or a new) group as part of the interactive deletion.
• Offering a separate bulk-reassignment menu to be used prior to deletion of a group.
• Adding in a warning dialog regarding the potential data loss prior of machines prior to deletion.

Describe alternatives you've considered
Just continue with the existing behavior.

Additional context
None.