Last year was a whirlwind for Reebok International Ltd., the athletic gear and footwear maker that moved its headquarters — and accompanying 750 employees — 18 miles from Boston’s suburbs into the city’s urban core. The Reebok team decided on a 220,000-square-foot space at Boston’s Innovation and Design Building in December 2016, signed the lease the following April, and began moving in September, said John Lynch, Reebok’s head of U.S.