Remove build artifacts and update build process

[HammadTheOne]

This PR removes build artifacts from the dev branch of dash and updates the build scripts. It also includes some cleanup for obsolete html components.

Contributor Checklist

• TODO

  • Update package.json scripts to a single npm build script to build renderer and components.
  • Update CONTRIBUTING.md
  • Update dash-core release process (https://github.com/plotly/dash-core/pull/336)
  • Remove obsolete html components.
  • Fix table helpers import issue (if in scope)

• I have run the tests locally and they passed. (refer to testing section in contributing)

optionals

• I have added entry in the CHANGELOG.md

[jonmmease]

I'll let @alexcjohnson give the final OK, but this looks good from my end.

Thanks @HammadTheOne!

[alexcjohnson]

💃 Love it! All sorts of good stuff here.