Build dev bundles in development

[ianb]

With #3432 we are no longer building hybrid development/production bundles, but instead build production builds all the time. It would be better if we built production bundles in production, and development builds in development.

To do this we need to add a flag to bin/build-scripts/bundle_dependencies for a dev or production build. A dev build doesn't uglify, uglifyify, and has NODE_ENV=dev. Then we should use bin/build-scripts/set_file to actually control the bundles, and make the generated file (the one that contains the flag value) a dependency for all bundles. Then we should make sure in bin/run-server it is set to dev, and in bin/release-version it is set to production.

[jaredhirsch]

Moving into 57 milestone, because non-debuggable JS is an obstacle for contributors

[chenba]

#3643 (PR of which has been merged) fixes part of this.