After Prop 1's victory last November, we celebrated the end of marijuana prohibition in Michigan. But the effort to move marijuana policy reform forward isn't over. The frontlines have now shifted to cities and towns, where many municipalities are imposing bans on marijuana businesses in their jurisdictions. It's not only about holding the line. Local activism opens up the possibility of more progress, too. Organizers in Ann Arbor, for example, are working to put a social use initiative on the ballot…