Allow for whitespace in factoring polynomials

Summary:
Noted here: https://github.com/Khan/khan-exercises/issues/158683

If the solution was of the form (-Ax + E), the answer would be rejected if
there was any whitespace between '+' and E.

Test Plan:
* Go to a problem with an answer of this form
  * exercises/factoring_polynomials_by_grouping_1.html?debug&problem=0&seed=175 is one
* Check that the answer '(5x + 7)(-x + 5)' is accepted

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D5983