Merge Request widget Improvements

What does this MR do?

A whole lotta MR Widget UI improvements.

• Even out padding/vertical spacing
• Status icons to the left of each section
• lines between sections

How to test

All components/states have been tested in mr-storybook branch Setup/test instructions on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12709

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

UI polish

Screenshots (if relevant)

most-states

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #33095 (closed)