Refactor storage code

[itsrachelfish]

storage.js is getting pretty big. It's almost 1000 lines, quite stinky.

The storage code handles several different aspects of working with savefiles:

• Reading savefiles from localstorage and user uploads
• Loading content onto the page
• Updating what content is on the page between slides
• Acting as the interface for the rest of the app to save, update, and delete content
• Controls when animations and media elements should play
• Controls when slides should tween

That's a lot of stuff for one file.