New product badges for GitLab.com plans

• For #1906 (closed) - STEP 2
• Closes #1904 (closed)
• Adds support for badges for GitLab.com plans (Free, Bronze, Silver, Gold)
• Available for release posts, features.yml, and comparison pages
• Merge !9645 (merged) before this MR

Review Apps:

• Release post (look for the first 5 secondary features): https://new-product-badges-dot-com.about.gitlab.com/2018/01/22/gitlab-10-4-released/#other-improvements-in-gitlab
• Features (look for "Built-in CI/CD"): http://new-product-badges-dot-com.about.gitlab.com/features/
• Comparison (look for "Built-in CI/CD"): http://new-product-badges-dot-com.about.gitlab.com/comparison/