The general assembly shall by general law delegate such legislative authority as from time to time it deems appropriate to towns, cities and boroughs relative to the powers, organization, and form of government of such political subdivisions. The general assembly shall from time to time by general law determine the maximum terms of office of the various town, city and borough elective offices. After July 1, 1969, the general assembly shall enact no special legislation relative to the powers, organization, terms of elective offices or form of government of any single town, city or borough, except as to (a) borrowing power, (b) validating acts, and (c) formation, consolidation or dissolution of any town, city or borough, unless in the delegation of legislative authority by general law the general assembly shall have failed to prescribe the powers necessary to effect the purpose of such special legislation.