Stack svg icons
There are different methodologies creating SVG sprites. We currently use
symbols which works really well with <use>
Unfortunatley one cannot
reference the images directly and thus not use it in CSS image URLs for
backgrounds for example.
This change stacks icons above each other, making the targeted one visible via CSS. It seems like it "just works", but hasn't been extensively tested in GitLab-Org/GitLab. And hey, it's even smaller.