Put all build commands in setup.py command classes

[agjohnson]

In the translation workflow, Babel adds helper scripts to setup.py, and #778 added some helpers to call all of the required translation pieces.

#771 uses a standard npm run to wrap around the webpack calls, however would it make more sense to also put these in setup.py? We could even extend the build command class to execute the webpack build prior to the rest of the packaging build steps.

• python setup.py build is consistent and fits with our release packaging. It keeps commands in a singular place
• npm run build implies that the build process requires npm and installation through npm
• Middle ground: python seutp.py build calls npm run build, it's all about choices.

Which are you most likely to use (or want to use)?

[jessetan]

I'm fine with using setup.py, but npm or yarn is still needed to pull in the dependencies.

[agjohnson]

Yup of course, I'm just referring to removing scripts from package.json.

A quick test shows python setup.py build is really nice, bdist and friends chain off of the Webpack build nicely.

I added a POC PR at #809 that shows this. The npm run dev command has waffled again to not collide with python setup.py develop. Suggestions welcome there.

[agjohnson]

Already merged. Commands are python setup.py build and python setup.py watch for now.