Better linting of icon/illustration svgs

We currently lint/plan to lint the following things:

• icons

  • no <mask> element used
  • no <defs> and <use> used: #38 (closed)
  • make a decision when it comes to: <title> and <desc>: #41 (closed)
  • icon size (bounding box): #40 (moved)

• illustrations:

  • enforce file name: #34 (moved)