Show modal with human-readable diff when "Preview Changes" is clicked.

[jonstout]

[jonstout]

Modal should display two buttons:

• Approve Changes
• Cancel

If diff has already been approved, the Approve Changes button should be disabled and text changed to Approved.

[jonstout]

• create modal to hold DiffApprovalForm component
• update getDiff in nodes.js to get diff text via node.cgi