SVG icon sprites

[rofe]

As originally mentioned by @davidnuescheler in #500, this enhancement request is for adding support for SVG sprites, so all icons of a site could be stored in a single SVG file and referenced by ID, e.g.:

Sprite URL: /icons.svg (configurable or by convention)

<svg class="icon icon-twitter">
    <use xlink:href="/icons.svg#twitter"></use>
</svg>

[rofe]

@davidnuescheler should we move this to helix-pipeline-service or close won't fix?