Use swimlanes sidebar for group boards [RUN AS-IF-FOSS]

What does this MR do?

• Enables the new (aka swimlanes) sidebar for group boards behind graphql_board_lists FF.

Here are currently known, expected problems:

• the epic dropdown won't always work when used in group boards.
• the milestone dropdown has issues regardless of where it is used (in group boards or epic swimlanes) #295626 (closed)

Screenshots (strongly suggested)

Screen_Recording_2021-01-14_at_6.39.56_PM

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

Issue #285074 (closed)