Add BaseBoardList superclass and refactor other resources to inherit from it

What does this MR do?

Create a BoardList superclass resource and refactor the LabelBoardList and MilestoneBoardList resources to inherit from it.

With this refactoring, we have less duplication and the same behavior with 4 fewer lines of code.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content