By Kristian Brunse and Sarah Mills LONDON (Reuters) – Performing their much-loved hits like “Mamma Mia!” and “Dancing Queen”, Swedish supergroup ABBA returned to the stage on Thursday, albeit as digital avatars, for a new London concert residency. The band – Bjorn Ulvaeus, Benny Andersson, Agnetha Faltskog and Anni-Frid Lyngstad – have been brought to virtual life as digital versions of themselves from their 1970s heyday, thanks to motion-capture technology. Their last performance together was some 40 years ago. The foursome, all now in their seventies, posed for pictures together at the conce…