Resolve "Scoping an Issue Board by label does not load filter" [RUN AS-IF-FOSS]

What does this MR do?

Behind the board_new_lists dropdown feature flag, selecting a label to add as a column doesn't work for large lists of labels.

Save selectedItem object rather than depending on items staying. items changes when we add filter, and filter value is cleared when label or other item is selected in the dropdown.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • [-] I have included a changelog entry.
  • I have not included a changelog entry because feature flag.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Related to #326270 (closed)