Allow answers to ignore the denominator when it's 1

Summary: Remove Y= in front of expressions to avoid confusion and accept more answer forms. For a request from Tomer.

Test Plan:
Make sure the exercises accept sensible answers.

e.g.
multiplying_and_dividing_rational_expressions_3.html?debug&problem=multiply&seed=20 should now accept 1 (and not just 1/1).
adding_and_subtracting_rational_expressions_3.html?seed=197&problem=expr-top&debug&lang=en should accept a - 3.
dding_and_subtracting_rational_expressions_5.html?debug&seed=25 now accepts (p^2 + 21p + 23)/(3(p^2 + 3p + 2)) and (p^2 + 21p + 23)/((3p + 3)(p + 2)), but not (p^2 + 21p + 23)/(3(p + 1)(p + 2)) sadly.

Also fixed adding_and_subtracting_rational_expressions_3.html?debug&problem=expr-top&seed=107
So the question does appear as (-k-2)/(10k) + 3, which makes the hints confusing.

Reviewers: emily, eater

Reviewed By: emily, eater

Differential Revision: http://phabricator.khanacademy.org/D14209