add dist:css step to prod-ify CSS file

[giladgray]

Fixes #612, fixes #2122

Changes proposed in this pull request:

• a few fixes in node-build-scripts:
  • add output option to sass-compile
  • add more main fields to assert-package-layout
  • add auto-fix string for file-header rule in tslint-config
• sass-dist script runs postcss with autoprefixer and discard-comments plugins
• add important comment at the top of each scss entry file (preserved in dist output)
• add .browserslistrc file instead of individual autoprefixer config

Reviewers should focus on:

blueprint-datetime.css   185K => 18K
blueprint-icons.css      856B => 988B *
blueprint-labs.css        47B => 162B *
blueprint-select.css      54K => 4.6K
blueprint-timezone.css    57K => 323B
blueprint.css            1.4M => 432K **
docs-theme.css           233K => 29K
table.css                313K => 42K

* - file size increase due to inline sourcemap
** - 70% filesize reduction!!!

[blueprint-bot]

autoprefixer 7 for hoisting

Preview: documentation | landing | table

[reiv]

This would fix #2122 as well. ✨

(Edit: for those that don't already use autoprefixer, I mean.)

[adidahiya]

nice! just small comments. I think the new script should be called css-dist

[adidahiya]

can we be more explicit? I don't really want to have to look at the documentation for the default. also more comments in this file would be really helpful, to explain its context

[adidahiya]

isn't this more like css-dist? it has nothing to do with sass.

[giladgray]

yarn cache issue with renamed bin script causing build fails

[adidahiya]

lol

[adidahiya]

that's annoying... let's merge into develop and fix things there if necessary