Whenever it is deemed advisable to do so by the city council thereof, any city having a corporate existence in this state at the time of the adoption of the Constitution thereof is hereby authorized and empowered to borrow money and to contract indebtedness in any other manner for general municipal purposes, not exceeding in amount, together with the existing general indebtedness of the city, the amount of indebtedness authorized by chapter
39.36 RCW, as now or hereafter amended, to be incurred without the assent of the voters.