[Feature Request] Make all js dependencies as assets

[byronz]

Is your feature request related to a problem? Please describe.
the dash-renderer repo includes both source code and binary assets/bundles for historical reasons. This is in general not good practices:

1. the repo is bigger than it's needed
2. the bundles and assets breaks the clean environment rule and make some development confusion/issues from time to time
3. it also makes the CI job slower and polluted

Describe the solution you'd like
a similar solution as proposed in plotly/dash-core-components#597

the repo should only contain source code, from the clean env, we do the build from a python script:

• run npm i --ignore-scripts
• parses the package-lock.json, copy the required bundles into dash_core_components, compute the content hash and dump all the fingerprint in a json file. this file can be included in the release as an alternative way than version to check the assets correctness.
• update __init__.py, so we have the single version of truth from npm
• run npm run build
  at this point, we should be able to dev and test from pip install -e .

apply the same procedure in circleci, and see how it applies for the release automation

Additional context
the script should work in both python2 and 3

[byronz]

@rpkyle @alexcjohnson in step2, if we compute all the bundle hash and dump then in a dict like

{ 'prop-types@15.7.2.js': '85947944e396a28895fad5f553eee36f', 'dash_renderer.dev.js': 'e6d5a2bb6733ecd334d8aa4369560afe' ...} 

this can be used as extra test in CI, or local development env whenever we have a doubt, just compare the content with the hash

[alexcjohnson]

Can we just use git to do the comparison for us? something like:

npm run build
git add dash-renderer/dash_renderer
git status -s dash-renderer/dash_renderer  # any output is a failure

[byronz]

this is like testing the .gitignore, we can add **/dash_renderer in the ignore file for that.

but the hash output is more like a reference when we release a dash version, we can tell if the bundles we built or copied from npm_modules are changed by git diff.