Ask for confirmation on all delete operations.

[drautb]

• When a user clicks delete on an item, it should ask them for confirmation, explaining that EVERYTHING will be gone.
• When a user clicks delete on a category, it should:
  • Verify that the category isn't being used. If it is, report which items are using it.
  • Ask for confirmation.
• When a user clicks delete on a unit, just delete it. They're cheap and easy to recreate.