(Reuters) - Online gambling firm 888 Holdings on Friday forecast full-year adjusted core earnings significantly ahead of expectations as more users came on board, sports events returned and its poker business performed well.Shares in the FTSE-250 company jumped 15.6% by 0837 GMT to a seven-month high of 174.4 pence and topped the midcap leaderboard.Gambling companies have seen a mixed impact from the coronavirus pandemic, which shut down one big cash cow - competitive sport - globally but..