PHILADELPHIA — If political conventions tell us anything beyond the predictable, the one held last week in Cleveland and the other held this week in Philadelphia pose contrasts so stark that one wonders if the two groups hail from the same country. Hint: One of them didn’t present a diverse cross-section of America. Whereas Cleveland’s arena was a relatively sparsely populated panorama of predominantly pale faces animated by anger, Philadelphia’s was a teeming, multicolored mass of (mostly) joyous celebration.