Build: Restore minification for IE8

- Use compatibility mode for minification, although this still produces
some incompatible CSS
- Move all "@" keywords to newlines so that when IE8 stops processing
the unknown CSS it doesn't drop other CSS on the line
- Bump cssmin to the latest version. Note the "^" selector is the new
default for NPM and needs later versions of NPM to understand.
Merge pull request #4750 from nschonni/v4-ie8-css-minification

Build: Restore minification for IE8