Reuse scope board mutation logic for epic boards

What does this MR do?

Label arguments and related logic (arguments validation) is needed for epic boards too, so these arguments are moved to ScopedBoardMutation and issue-specific arguments are renamed left in ScopedIssueBoardArguments.

UPDATE: this MR also fixes GraphQL response of board create/update mutations if creation fails - we should return nil instead of invalid/unsaved object (which would fail on create anyway because it couldn't be properly rendered).

Related to #323370 (closed)

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