Many companies have hired editorial staffers in recent years to write content for their blogs and be their voices on social media. Now, some are going further by building full-blown media properties of their own. Take startup mattress brand Casper, for example. The company is currently hiring journalists and gearing up to start its own standalone Web publication about sleep. The site doesn’t have a name or a URL yet, but it’s slated to launch later this spring. “Sleep is a growing subject that lacks a true editorial authority,” said Lindsay Kaplan, Casper’s vice president of communications.