A slain Brooklyn slumlord’s shady business practices helped sink a bank that counted President Obama as a customer and ex-Chicago Bears quarterback Jim McMahon as a member of its board, The Post has learned. Two unpaid loans to murder victim Menachem “Max” Stark and his business partner Israel “Sam” Perlmutter were among 17 bad bets that spelled $104 million in losses for Chicago’s Broadway Bank before it was shut down by the feds in 2010, court papers show. [BIG SNIP] Broadway Bank was run by Demetris and George Giannoulias, brothers of former Illinois state Treasurer Alexi Giannoulias. Alexi is a...