Update all of AMP's CSS devDependencies to their latest versions

[rsimha]

This PR does the following:

1. Updates all CSS related devDependencies via yarn add autoprefixer cssnano postcss postcss-import --dev --exact --latest
2. Manually updates cssnano from v3.10.0 to v4.0.0-rc.2 to get past a deprecation warning for Node#moveTo. (See ben-eb/postcss-merge-rules#52)
3. Updates some rules in build-system/tasks/create-golden-css/index.js and build-system/tasks/jsify-css.js to be compliant with cssnano v4, with minimum deviations from the CSS generated pre-upgrade.

This can be followed up with a PR that updates our cssnano settings and uses some of the new optimization features in v4.

Follow up to #12179, #12450, #12476, #12486, and #12499
Partial fix for #12181

[rsimha]

/to @erwinmombay

This is as discussed in #12472 (comment)

[rsimha]

@erwinmombay This PR passes Travis checks. Let me know if there's any documentation for the golden file check, and I can run it tomorrow.

[rsimha]

CSS golden file check coming up in #12526.

[rsimha]

@erwinmombay I tested the latest changes with gulp create-golden-css and generated a new golden file after updating some options. We are now as close to the original cssnano config as possible. The exceptions being what's in https://github.com/ampproject/amphtml/pull/12512/files#diff-f30964a2accb052b4e62450f31255bb1, all of which you reviewed and approved.

Merging this after tests go green.

[rsimha]

@camelburrito @erwinmombay After this PR is merged, it's probably worth updating our cssnano settings and using some of the new optimization features in v4.

Full list of available packages here: https://github.com/ben-eb/cssnano/tree/master/packages