Reuse minified JS from previous deploys

This is a big change affecting lots of areas:

* Pipeline no longer deals with JS (though it still minifies CSS)
* A new script, tools/minify-js (called from update-prod-static),
  minifies JavaScripts
* A command-line argument --prev-deploy, if passed to minify-js or
  update-prod-static, is used to copy minified JS from a previous
  deploy (i.e., a previous git checkout), if the source files have
  not changed
* update-deployment passes --prev-deploy
* Scripts are now included with the minified_js template tag, rather
  than Pipeline's compressed_js

Also, as a side benefit of this commit, our Handlebars templates will
no longer be copied into prod-static/ and accessible in production.

Unminification is probably broken, but, per Zev and Trac ticket #1377,
it wasn't working perfectly before this change either.

(Based on code review, this commit has been revised to:
 * Warn if git returns an error in minify-js
 * Add missing output redirects in update-prod-static
 * Use DEPLOY_ROOT instead of manually constructing that directory
 * Use old style formatting)

(imported from commit e67722ea252756db8519d5c0bd6a421d59374185)

zephyr/lib/unminify.py

@@ -19,7 +19,7 @@ class LineToFile(object):
         self._cumulative_counts = []
 
         total = 0
-        for filename in settings.PIPELINE_JS['app']['source_filenames']:
+        for filename in settings.JS_SPECS['app']['source_filenames']:
             self._names.append(filename)
             self._cumulative_counts.append(total)
             with open(path.join('zephyr/static', filename), 'r') as fil: