Saving / New App UI Updates - fixes #1787

STT
* [ ] **New App* button is now inside the hamburger menu
* [ ] **Save App* button is now outside the hamburger menu
* [ ] Hit **New App** when on an *Unsaved App* that you've added some bricks to
  * You'll see a warning prompt about losing your work.

This change prevents the accidental loss of all your work on an Unsaved App when New App. It also puts Save in a more prominent place. I'll file a follow up issue about auto-saving progress on an app as you work.