Release post: reference buttons - add product icons

• Closes #2482 (closed)
• Adds GitLab product icons to reference buttons on the release post:

Preview

Review app: https://release-post-reference-buttons-update.about.gitlab.com/2018/07/22/gitlab-11-1-released/

Top & primary features

https://release-post-reference-buttons-update.about.gitlab.com/2018/07/22/gitlab-11-1-released/#security-dashboard-for-projects

Secondary features

https://release-post-reference-buttons-update.about.gitlab.com/2018/07/22/gitlab-11-1-released/#contribution-analytics-redesign

Performance improvements

https://release-post-reference-buttons-update.about.gitlab.com/2018/07/22/gitlab-11-1-released/#performance-improvements

Checklist

• Jarek's review
• Revert tests
• Update 11.2 MR (!13265 (merged))