Avoid inlining SVGs in CSS

Problem/Motivation

Admin theme compiles SVGs inline into its CSS, see #3623385: Admin theme's status report styling is off.

This has a couple of downsides:

- cache hit rate is lower - SVGs are guaranteed to only be loaded then cached in the browser, but CSS sometimes gets duplicated between multiple aggregates

- SVGs are loaded on pages where they don't appear, because there is a single icons CSS file covering icons that may only be used by a conditionally loaded library

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet