Allow a user to skip a question shortly after getting it wrong

Test Plan:
Answer an answer incorrectly in a mastery task
Click skip within 1000ms (without changing the answer-text!)
See that you can skip the question

Before, users were effectively stuck on this problem because the skipping caused canAttempt to false, but then the conditional in this changeset (before this diff) would prevent the skip from actually happening.

Reviewers: jack

Reviewed By: jack

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D7419