Section 2. Except as otherwise expressly permitted by law, cities, towns and districts shall incur debts only in the manner of voting and within the limitations as to amount and time of payment prescribed in this chapter; but this section shall not be construed as prohibiting any city, town or district from placing additional restrictions, consistent with this chapter, upon the manner of incurring debt, nor as affecting the right of any city, town or district to incur debt under any special act which has or shall become effective after January first, nineteen hundred and twenty-one, or at any time in the case of debt of the city of Boston for tunnel or transit purposes; but no debt may be authorized under a general or special act except by a two-thirds vote, unless the act so provides. All provisions of law providing for sinking funds for the payment of debt of the city of Boston incurred for transit or tunnel purposes shall continue to be applicable to said debt.