Align radio checkboxes with center of table

Summary: Align radio checkboxes with center of table to fix T2534. Alternatively, we can style `perseus-radio-option-content` to vertically-align all checkboxes to the center of the radio content, in case we want to be more consistent. (Decided against for now, since centering on paragraphs looks bad.)

Test Plan:
Open http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%2A%2AHow%20do%20we%20count%20the%20mice%3F%2A%2A%20%20%5Cn%5Cn%5B%5B%E2%98%83%20radio%201%5D%5D%22%2C%22images%22%3A%7B%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F091be90c14102988059b4e2e13a14a897b629f83.png%22%3A%7B%22width%22%3A50%2C%22height%22%3A50%7D%7D%2C%22widgets%22%3A%7B%22radio%201%22%3A%7B%22type%22%3A%22radio%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22choices%22%3A%5B%7B%22content%22%3A%22%21%5B%5D%28https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F86b398d9f8fd05dd769e6f35a8d1c68a8bb23ff9.png%29%22%2C%22correct%22%3Afalse%7D%2C%7B%22content%22%3A%22%21%5B%5D%28https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F86b398d9f8fd05dd769e6f35a8d1c68a8bb23ff9.png%29%20%7C%21%5B%5D%28https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F86b398d9f8fd05dd769e6f35a8d1c68a8bb23ff9.png%29%20%7C%20%21%5B%5D%28https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F86b398d9f8fd05dd769e6f35a8d1c68a8bb23ff9.png%29%20%7C%20%21%5B%5D%28https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F86b398d9f8fd05dd769e6f35a8d1c68a8bb23ff9.png%29%20%7C%20%21%5B%5D%28https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F86b398d9f8fd05dd769e6f35a8d1c68a8bb23ff9.png%29%20%7C%20%21%5B%5D%28https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F86b398d9f8fd05dd769e6f35a8d1c68a8bb23ff9.png%29%20%7C%20%21%5B%5D%28https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F86b398d9f8fd05dd769e6f35a8d1c68a8bb23ff9.png%29%5Cn%3A-%3A%20%7C%20%3A-%3A%20%7C%20%3A-%3A%20%7C%20%3A-%3A%20%7C%20%3A-%3A%20%7C%20%3A-%3A%7C%3A-%3A%5Cn%242%24%7C%243%24%7C%244%24%7C%245%24%7C%246%24%7C%247%24%7C%248%24%5Cn%24%24%22%2C%22correct%22%3Afalse%7D%2C%7B%22content%22%3A%22Lorem%20ipsum%20dolor%20sit%20amet%2C%20consectetur%20adipiscing%20elit.%20Vivamus%20mi%20dolor%2C%20accumsan%20et%20ultricies%20sed%2C%20fringilla%20ut%20nibh.%20Nulla%20sed%20congue%20lacus.%20Interdum%20et%20malesuada%20fames%20ac%20ante%20ipsum%20primis%20in%20faucibus.%20Vestibulum%20id%20mattis%20sem.%20Maecenas%20consectetur%20nibh%20nec%20metus%20ultricies%2C%20nec%20vehicula%20dui%20dictum.%20Donec%20vel%20tortor%20velit.%20Sed%20hendrerit%20urna%20elit%2C%20eleifend%20vehicula%20velit%20ornare%20eu.%20Suspendisse%20potenti.%20Fusce%20ac%20eros%20est.%20Donec%20viverra%20sed%20turpis%20a%20posuere.%20Vivamus%20ornare%20vestibulum%20urna%2C%20non%20tempus%20erat%20varius%20sed.%20Donec%20rutrum%2C%20erat%20at%20euismod%20luctus%2C%20lorem%20massa%20euismod%20tortor%2C%20nec%20pretium%20tortor%20ante%20vitae%20neque.%22%2C%22correct%22%3Atrue%7D%5D%2C%22randomize%22%3Afalse%2C%22multipleSelect%22%3Afalse%2C%22displayCount%22%3Anull%2C%22noneOfTheAbove%22%3Afalse%2C%22onePerLine%22%3Atrue%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22Select%20the%20correct%20answer.%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22itemDataVersion%22%3A%7B%22major%22%3A0%2C%22minor%22%3A1%7D%2C%22hints%22%3A%5B%7B%22content%22%3A%22Count%20like%20this%3A%20%20%5Cn%5Cn%5Cn%21%5B%5D%28https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F86b398d9f8fd05dd769e6f35a8d1c68a8bb23ff9.png%29%20%7C%21%5B%5D%28https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F86b398d9f8fd05dd769e6f35a8d1c68a8bb23ff9.png%29%20%7C%20%21%5B%5D%28https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F86b398d9f8fd05dd769e6f35a8d1c68a8bb23ff9.png%29%20%7C%20%21%5B%5D%28https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F86b398d9f8fd05dd769e6f35a8d1c68a8bb23ff9.png%29%20%7C%20%21%5B%5D%28https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F86b398d9f8fd05dd769e6f35a8d1c68a8bb23ff9.png%29%20%7C%20%21%5B%5D%28https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F86b398d9f8fd05dd769e6f35a8d1c68a8bb23ff9.png%29%20%7C%20%21%5B%5D%28https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F86b398d9f8fd05dd769e6f35a8d1c68a8bb23ff9.png%29%5Cn%3A-%3A%20%7C%20%3A-%3A%20%7C%20%3A-%3A%20%7C%20%3A-%3A%20%7C%20%3A-%3A%20%7C%20%3A-%3A%7C%3A-%3A%5Cn%241%24%7C%242%24%7C%243%24%7C%244%24%7C%245%24%7C%246%24%7C%247%24%22%2C%22images%22%3A%7B%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F86b398d9f8fd05dd769e6f35a8d1c68a8bb23ff9.png%22%3A%7B%22width%22%3A45%2C%22height%22%3A45%7D%7D%2C%22widgets%22%3A%7B%7D%7D%5D%7D
Observe that the image and table options are centered, while the paragraph remains top-aligned.

Reviewers: alpert, aria

Reviewed By: alpert, aria

Subscribers: alpert

Maniphest Tasks: T2534

Differential Revision: http://phabricator.khanacademy.org/D11967