Should we be committing yarn.lock?

[chrishavlin]

#110 was failing in CI related to no yarn.lock being checked in, so in that PR I modified the CI env variable enableImmutableInstalls to allow yarn.lock generation during install. Should figure out what the current standard for jupyter widgets is -- may be that we should be checking it in.

[chrishavlin]

one point of info, the ts cookiecutter widget .gitignore only has the following

# NPM
# ----

**/node_modules/
{{ cookiecutter.python_package_name  }}/nbextension/index.*

# Coverage data
# -------------
**/coverage/

# Packed lab extensions
{{ cookiecutter.python_package_name  }}/labextension