Building yarn package on install

[cabreraalex]

I was curious if people know why the setup.py builds the package on install:

widget-ts-cookiecutter/{{cookiecutter.github_project_name}}/setup.py

Line 60 in f8c4f1d

install_npm(HERE, build_cmd='build:prod'),

It looks like nbextension and labextension are both pre-built and bundled, so it isn't necessary to run npm and build:prod on install. This has also been breaking for people who do not have yarn or npm installed.

[cabreraalex]

On a related note, it looks like jupyter_packaging does not support the sdist packaging in the docs:
https://github.com/jupyter/jupyter-packaging/blob/master/README.md#usage-notes

[blink1073]

A short term fix is to us skip_if_exists to skip the npm build if one of the built files exists. A better solution is to adopt the jupyter_packaging backend in this cookiecutter.

[cabreraalex]

Fix at #107