use horizontal dividers for MR widget sections

What does this MR do?

todo:

• double padding on helptext ("merge this branch manually"). Remove bottom margin
• check other items/components for border-top
• check padding of all inner elements ok
• check responsive

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

Why was this MR needed?

precursor to https://gitlab.com/gitlab-org/gitlab-ce/issues/33982

Screenshots (if relevant)

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 #33976 (closed)