Add CI Task to verify newly added icons don't contain <defs> and <use> blocks

As described here and in #37 (closed), <defs> and <use> blocks in our SVG icons lead to problems in IE 11 as svg4everybody checks every element that is a descendant of an <svg> and replaces it with a chunk of svg code. Per default, svg4everybody will not exclude inline <use> blocks which were used in the majority of our SVG icons.

We already took care of removing these blocks from our existing SVG icons in !176 (merged).

We should also add a CI task to ensure that newly added icons don't contain <defs> and <use> blocks.