SAN FRANCISCO (Reuters) - The chief operating officer for mobile payments company Square, Keith Rabois, abruptly resigned this week because an employee accused him of sexual harassment and demanded "millions of dollars" to stave off a potential lawsuit, Rabois said in a Friday blog post. The revelation by Rabois, a well-known senior executive and active startup investor, caused a sensation in Silicon Valley and ignited a flurry of online chatter about Square, founded in 2009 by Twitter co-creator Jack Dorsey. ...