Remove csslab dependency

Summary

The issue is created as a follow-up of this comment

Improvements

As csslab feature is deprecated we should clear everything related to it:

• remove the dependency from package.json
• remove csslab from the asset pipeline in config/application.rb
• remove the stylesheet link from app/views/layouts/_head.html.haml
• remove any references of 'use-csslab' if Feature.enabled?(:csslab)
• remove any references of stub_feature_flags(csslab: false)
• remove any references of .use-csslab in our css

Risks

Styling issues? anything else?

Involved components

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