Initiative and referendum powers are hereby reserved to the people of the City on all questions which the City may be authorized by this Charter, by ordinance of the City, or by general laws of the State, to control by legislative action, and such powers shall be exercised in the manner prescribed by general law of the State. No measure initiated by the people and adopted by popular vote shall be repealed by Council, or so amended by it as to destroy the effectiveness thereof, within two years after it takes effect.