Change "Interactive Scenario" to "Website Content" or similar

[matt-cook]

@ericholmer, @matt-schuh, @kstangel any ideas on what label makes sense? I'd like to move away from "Scenario" as the label as start referring to content nodes as "Content", since we've removed the extra layer of abstraction and content nodes are now scheduled directly. For example, an instance of "Slideshow Scenario" called "Cute Kittens" would now be an instance of "Slideshow Content" node type.