Use expression editor with polynomial exercises

Modify adding_and_subtracting_polynomials and multiplying_polynomials. Also add a function to polynomials util to return a string of the polynomial that the expression editor can interpret.

For Trello card: https://trello.com/c/DDGyWhUE/351-modify-existing-exercises-adding-and-subtracting-polynomials-multiplying-polynomials