Use routes for code and preview

[benjreinhart]

• Default /apps/:id to the preview tab
• Add two routes:
  1. /apps/:id/files (if user has last file opened saved in local storage, redirect to that. Otherwise, show empty state)
  2. /apps/:id/files/:path (show the file in the editor)

Lots of changes required to untangle things and make them configurable starting at the router.