Introduce board multi select feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

Merged Eulyeon Ko requested to merge introduce-board-multi-select-feature-flag into master

What does this MR do?

The refactored boards, which will roll out in this milestone, doesn't support multi select drag&drop.

Users will get confused when they can multi select issue cards but cannot use drag&drop so we decided to temporarily disable multi select by putting it behind a feature flag.

For more context, please see #289797

The new version of boards uses Draggable to drag and drop an issue.

Once Draggable supports multi drag (https://github.com/SortableJS/Vue.Draggable/pull/744), it should be added to GraphQL boards.

Does this MR meet the acceptance criteria?

Conformity

Edited by Eulyeon Ko