AT&T summoned the big players from the worlds of media and advertising to a beachside resort in Santa Barbara. The company had finally closed its $85 billion acquisition of Time Warner, after a protracted fight with the Department of Justice, and wanted to celebrate — and to mark its place at the center of the media ecosystem.The three-day AT&T Relevance Conference, at the lavish Ritz-Carlton Bacara, featured appearances by Derek Jeter and Issa Rae, as well as AT&T CEO Randall Stephenson.