Add board_new_list feature flag and placeholder [RUN AS-IF-FOSS]

What does this MR do?

Phase 0 of Move create list to new column widget.

Add feature flag and replace 'Add list' dropdown with the button that will trigger the column to appear.

The button doesn't actually do anything yet, so the feature flag is disabled in relevant rspec tests

Screenshots (strongly suggested)

before (or FF disabled)	after (with FF enabled)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content