Add a "Save and close" button to the editor popup

The user will now have to go to File -> Save before clicking Close on the popup in studio. This is not desirable. It is possible to add buttons to the edit screen. Add a button next to the Close button that saves the graph and closes the popup.

Bonus points for making sure the newly saved graph is present in the preview of the XBlock after closing the popup, but if that's difficult we should make that a separate issue.