Append .erb to ERB files so they syntax highlight better

[calebmeyer]

Summary

Our ERB files currently end in .json and .html. If we rename them to .json.erb and .html.erb then Atom, Github, and other places that display the code will be more likely to get the syntax highlighting right.

Motivation

Github in particular highlights ERB within JSON files as red:

Describe alternatives you've considered

You could name the files just .erb instead of .json.erb, but this seems like we're trading one problem (JSON highlighting ERB wrong) for another (ERB highlighting JSON wrong)

Additional context

Originally asked for on #541