Core outcome set for preventive intervention trials in chronic and episodic migraine (COSMIG): an international, consensus-derived and multistakeholder initiative by Haywood, K, Potter, R, Froud, R, Pearce, G, Box, B, Muldoon, L, Lipton, R, Petrou, S, Rendas-Baum, R, Logan, A-M, Stewart, K, Underwood, M, Matharu, M