Make issue boards height calculation clearer and remove magic numbers

What does this MR do?

Cleans up issue boards height calculation.

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

Why was this MR needed?

This relates to https://gitlab.com/gitlab-org/gitlab-ee/issues/2680 and the corresponding MR https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4944

Screenshots (if relevant)

This screencast shows the issue board behavior for different screen sizes. In order: MD+, SM, XS. Then I open focus mode (EE only) 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
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?