Exit package creator: use React dialogue to confirm exitting if changes have been done

[cpina]

Steps to reproduce:

• Login as a manager to schema-collaboration
• On a datapackage click on Options -> Edit datapackage
• Make changes (e.g. change the name of the datapackage)
• Click on "Save to Server"
• Make changes (e.g. change the name again)
• Click on "Exit datapackage creator"

A dialogue will appear that reads: "If you continue you will lose the changes in this session" [Ok, Cancel]

This dialogue is a "confirm" in Javascript. It should be some nicer dialogue in React.