Yoko Ono Lennon will celebrate 50 years of the John Lennon/Plastic Ono Band — a record Lennon described as "the best thing I’ve ever done" — with the release of a massive eight-disc super-deluxe box set. The 159-track, 11-hour collection, which boasts 87 never-before-heard recordings, arrives April 16 via and Capitol/UMe. The ultimate mix of "Mother" launches the…