This change switches us over to using pipeline's compilers all the time, not

just when PIPELINE_ENABLED = True, as well as adding the babel compiler to

the list. This means that compilation of LESS (and ES6!) happens on the

back-end instead of the front-end during development.
This is better than before because instead of recompiling all the stylesheets

on page load, we do it on-demand, and pipeline does mtime checks to verify

whether it needs to actually do any work. This significantly speeds up reloads

during development.
This adds new dev dependencies on lessc and babel. These are included in a

package.json file that allows one to just run "npm install" to get everything

necessary.

Verified that recompile on-demand from within the devserver worked as

  expected.
Converted common.js to common.es6 and made use of some ES6 features. Verified

  that what I got in the browser was transpiled ES5.
Converted a JS file in an extension to ES6 and saw similar results.