Consider adopting a third party CSS minification library

Problem/Motivation

Following #1014086: Stampedes and cold cache performance issues with css/js aggregation asset aggregation no-longer blocks HTML page requests, this means we can start to look at more robust minification methods.

This has come up lots of times before, but starting a new issue since most of the existing ones are very outdated at this point.

Last time we looked at this, the front-runner was Assetic. This is no longer developed by the original maintainer, but there's a maintained fork at https://github.com/assetic-php/assetic

Also found: https://github.com/matthiasmullie/minify

And: https://doc.wikimedia.org/mediawiki-libs-Minify/master/, which the new Assetic has as a dependency.

JavaScript has its own set of challenges and therefore its own issue #3302755: On-the-fly JavaScript minification.

Steps to reproduce

Proposed resolution

Remaining tasks

This is only worth doing if it ends up with a better result than what we currently have - both in terms of maintenance burden and output.

• Collect existing bug reports with CSS aggregation (there are a handful), and ideally come up with a CSS file that includes test-cases.
• Check the test case against the above libraries
• Dependency evaluation of the libraries
• Integration patch to include the library in core
• Potentially profiling against the existing implementation

User interface changes

API changes

Data model changes

Release notes snippet