build: Update dependencies & devDependencies to latest (#258)

• build: Update dependencies & devDependencies to latest autoprefixer 9.6.1 → 9.6.5 eslint-config-wikimedia 0.13.1 → 0.15.0 grunt-stylelint 0.11.0 → 0.12.0 postcss 7.0.17 → 7.0.18 postcss-custom-properties 8.0.10 → 9.0.2 stylelint-config-wikimedia 0.6.0 → 0.7.0

• Actually update package-lock

• Make eslint happy by dropping unused disable

• Make eslint faster with cacheing

• Gruntfile: Drop removal of path that isn't in the glob anyway

• Use normal syntax for eslint rule severity over-ride