added focus mode button to issue boards

What does this MR do?

Adds a button to initiate focus mode on issue boards. Focus mode hides all the UI that isn't directly related to issue boards making it ideal for showing issue boards on bigger screens without any additional UI.

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

Why was this MR needed?

This improves issueboards for display on big screens or other instances where navigation isn't needed.

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 #25234 (closed)