Resolve "IE 11 can't load SVG files"

Most SVG icons don't show up in IE11. The problem is caused by svg4everybody as it will check every element that is a descendant of an <svg> and replaces it with a chunk of svg code (as described here)

The majority of our SVG icons use inline svg source (with <defs> and <use> blocks) like https://gitlab.com/gitlab-org/gitlab-svgs/blob/master/sprite_icons/issues.svg

Per default, svg4everybody will not exclude these inline <use> blocks, see https://github.com/jonathantneal/svg4everybody/issues/92

This MR removes <defs> and <use> from our SVGs in order to avoid this problem. We are creating <symbol>s when building our svg sprite anyway which makes the use of <defs> unnecessary

Closes #37 (closed)