Add "last edited" date to all tutorials, on first slide

[khanspers]

Add "last edited" date to all tutorials, on first slide

[khanspers]

After some consideration, I'm not sure this is actually helpful. It might look worse to have an old date than no date at all.....
I added a footer updated class to cytoscape-presentation-1.css which can be used if someone wants to add a last updated date.