Styling of the MR widget's info and pipeline sections

This issue is a follow up to gitlab-ce#37479. It is part of the effort to improve the readability and overall usability of the MR widget. In this issue, we are styling only the info, pipeline and review apps sections of the MR widget. The remaining sections (security products, approvals, and merging) will be styled in later issues #46796 (moved) and #46797 (moved).