It shall be the duty of the legislature, subject to the provisions of this constitution, to restrict the power of taxation, assessment, borrowing money, contracting indebtedness, and loaning the credit of counties, cities, towns and villages, so as to prevent abuses in taxation and assessments and in contracting of indebtedness by them. Nothing in this article shall be construed to prevent the legislature from further restricting the powers herein specified of any county, city, town, village or school district to contract indebtedness or to levy taxes on real estate. The legislature shall not, however, restrict the power to levy taxes on real estate for the payment of interest on or principal of indebtedness theretofore contracted.