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: gitlab-org/gitlab-svgs#38 (closed)
  • make a decision when it comes to: <title> and <desc>: gitlab-org/gitlab-svgs#41 (closed)
  • icon size (bounding box): gitlab-org/gitlab-svgs#40 (moved)

• illustrations:

  • enforce file name: gitlab-org/gitlab-svgs#34 (moved)