The way we do a lot of multi-line templates in JavaScript code is pretty ugly,

building a big list of strings and then joining them. ES6 template strings are

a potential solution, but they introduce a lot of ugly whitespace if there's

any indentation in them.
This change adds babel-plugin-dedent to our build, which adds a preprocessing

step that removes leading indentation from template strings which are marked

with the dedent tag. This looks like an ordinary tag, but it's processed

during the compilation step instead of at runtime.

Ran setup.py develop and saw babel-plugin-dedent get installed correctly.
Used the dedent tag in extension code.