Issues: no results in issue search on Boards view when search criteria is 1 or 2 letters (problem not present on standard issue lists)

Summary

• many issues found on: https://host/group/project/issues?scope=all&utf8=%E2%9C%93&state=opened&search=s3
• zero issues found on: https://host/group/project/issues/-/boards/2?scope=all&utf8=%E2%9C%93&state=opened&search=s3

The board itself does not have any label filters so search results should be the same.

Proposed labels: issues issue list ~"Category:Issue Boards" Category:Issue Tracking . CC @markglenfletcher

Changes behind a feature flag

Given the likely performance costs, the feature flag board_search_optimization was implemented to test before release. See !23366 (comment 282082400)

The default value is false