DETROIT — A federal magistrate judge Monday refused to release on bond the former keyboardist for the Detroit rock band Mac Saturn, citing the “extreme danger” he poses to the community in a child pornography case that alleges the musician masturbated online with three minors. A lawyer for keyboardist Evan Mercer, 30, of Farmington, vowed to appeal after Chief U.S. Magistrate Judge David Grand ordered the musician remain in jail while awaiting trial on charges that could send the rocker to federal prison for up to 30 years. The detention hearing came three days after court officials unsealed a…