Fix & enable "JS Chrome app (using Polymer)" + fix other templates

@devoncarew: just a nominal review, there is not much actual app code that's changed.

I'll submit a follow-up PR, which will add a warning to run the CSP Fixer after creating a "JS Chrome app (using Polymer)" and finally move it from under a flag.
Why this change from index.html and main.dart? I prefer the previous names.