The grantee of a franchise may not sell, assign, sublet, or allow another to use the same, unless the Council consents. Nothing in this section shall limit the right of the grantee of any public utility franchise to mortgage its property or franchise, nor restrict the right of the purchaser, upon foreclosure sale, to operate the same, except that such mortgagee or purchaser shall be subject to the terms of the franchise and the provisions of this Charter.