Add DevOps stages icons to release posts

• Follows !17198 (merged)

• Adds stage icons to represent the DevOps stage a feature belongs to:

  

• Shows the stage name on hover, linking to each stage page.

Review apps:

• Post: https://rename-team-to-stage-for-release-post-items-add-badges.about-src.gitlab.com/2018/11/22/gitlab-11-5-released/
• Handbook update: https://rename-team-to-stage-for-release-post-items-add-badges.about-src.gitlab.com/handbook/marketing/blog/release-posts/#stage