N. Vidi and . Grant, P. L. acknowledges the support of the International Max Planck Research School Bonn/ Cologne. K. L. acknowledges the financial support by the European Research Council for the ERC Synergy Grant BlackHoleCam under Contract No. 610058. S. O. is supported by the Alexander von Humboldt Foundation, The authors also acknowledge support of NSF Award No. PHY- 1066293 and the hospitality of the Aspen Center for Physics

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