Release posts: add support for dark cover image

• Closes #2392 (closed)
• Add layout for dark cover images header_layout_dark: true
• Add support for choosing the release number color (light/dark) release_number_dark: true
• Clean up the code
• FE review
• Remove tests/examples (11.1, 10.8)

Review app:

• Dark cover image: https://release-post-support-dark-cover.about.gitlab.com/2018/07/22/gitlab-11-1-released/:

  

• Dark release number: https://release-post-support-dark-cover.about.gitlab.com/2018/05/22/gitlab-10-8-released/: