Issue Boards: Ensure that horizontal scroll bars are shown on overflow

What does this MR do?

• Clean up calculation of board-lists height
• Ensure that horizontal scroll bars are shown on overflow

Why was this MR needed?

See reasoning in #2680 (closed)

Screencast:

https://youtu.be/349hvD6s6Ig

The cast shows the issue board behavior for different screen sizes. In order: MD+, SM, XS. Then I open focus mode and show how board behaves. If you look closely at the horizontal bar at the bottom, it's always there!

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #2680 (closed)