Optimize CSS loaded with `@StyleSheet`

[sissbruecker]

Describe your motivation

@StyleSheet currently just adds a link tag to the document head, but does no further optimization on the CSS. When building large applications that rely on a lot of custom styling, or include a custom base theme, this can result in several issues:

• The CSS that needs to be loaded by the browser is not optimized for size
• When splitting up stylesheets into multiple files for better organization and importing them from a main stylesheet with @import, this results in a "waterfall" of smaller requests which results in the app taking longer to render

Since @StyleSheet is intended to become the default solution for loading styles in v25 we should look in to improving this.

Describe the solution you'd like

The CSS loaded through @StyleSheet should be optimized for production:

• The CSS should be minified to reduce the amount of data transferred to the browser
• The CSS should be inlined so that nested imports do not result in separate requests to the server

In dev mode such an optimization is probably not necessary.