Add stack-rendered sprite to dist

It would make sense to switch to stacked rendering because it allows us to re-use the sprite icon in CSS, but we had issues with it in the past.

By adding a separate dist icon sprite we are able to switch and test everything properly rather than breaking things.

See: gitlab-com/gl-infra/production#2881 (closed)

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You can see that support should be there in all browsers we care about, so I assume that the issues like gitlab-com/gl-infra/production#2881 (closed) can be fixed with CSS.

Added a page that shows rendering should be fine on the major browsers: http://leipert-different-sprites.64.227.120.153.nip.io/gitlab-svgs/render_tests

Browser	Screenshot
Firefox 85
Chromium 85
Safari 14