make all dependencies as assets

[alexcjohnson]

We should be able to treat the plotly.js file as a build artifact, such that upgrading it is as simple as:

• update the version number in package.json -> devDependencies
• add a changelog note
• do NOT commit changes in the dash_core_components dir

Then its build step would consist of:

• read the version out of node_modules/plotly.js/package.json (don't get it from our own package.json, in case npm i hasn't been called)
• delete old dash_core_components/plotly-*.js
• copy node_modules/plotly.js/dist/plotly.min.js into dash_core_components/plotly-<version>.min.js
• update dash_core_components/__init__.py

Aside from simplifying updates, this would prevent various possible errors, and ensure py/R consistency (plotly.js is already treated as a build artifact as far as the R package is concerned)

[byronz]

@alexcjohnson , chatted with @rpkyle today, I will take this issue as placeholder for plotly.js and also hightlight.js and css, the current repo has replicated js bundles, and the R script copied the bundles from the dash_core_component repo, ideally, we should have a common assets folder for all languages, a symbolic link might be the way for that, I will investigate and update the status here