Previously on the all-new DC Universe: With last year's Superman, James Gunn admirably rebooted Warner Bros' big-screen superhero canon that kept putting the "League" in "beleaguered" with every new misfire. Rather than fully suppress the bad-boy edginess of his Guardians of the Galaxy trilogy -- which worked for that particular motley crew, don't get me wrong -- Gunn shrewdly tapped humankind's self-inflicted age of cynicism as a chief antagonist to the Big Blue Boy Scout's superpowered optimism. He wasn't necessarily renouncing "attitude" as a marketing mood, so much as he was conceding there…