Added auto-none-of-the-above functionality to Radio widget

Summary:
Radio widget now allows for an auto-none-of-the-above option, where it
randomly chooses one answer to omit and replace with "None of the above", as is
often done in khan-exercises. In the process, much of the randomization logic
was refactored out to a prop transform between editor and widget, and several
props were removed from the 'choices' in the Radio component, helping to remove
some information to which the Radio should not have had access.

Test Plan:
- Open up a single-choice Radio widget ([shortcut](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5Cn%5Cn%5B%5B%E2%98%83%20radio%201%5D%5D%22%2C%22images%22%3A%7B%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%22A%22%7D%2C%7B%22content%22%3A%22B%22%7D%2C%7B%22content%22%3A%22C%22%7D%5D%2C%22randomize%22%3Afalse%2C%22multipleSelect%22%3Afalse%2C%22displayCount%22%3Anull%2C%22noneOfTheAbove%22%3Atrue%2C%22onePerLine%22%3Atrue%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22hints%22%3A%5B%5D%7D)). Things to check:
    - When the correct answer is replaced with 'None of the above', 'None of the above' becomes the new correct choice.
        - When this answer is accepted, the underlying correct answer is displayed. This disappears if you choose a new answer and re-grade (TODO: figure out this exact functionality).
    - In all other cases, 'None of the above' is wrong.
- Open up a multiple-choice Radio widget ([shortcut](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5Cn%5Cn%5B%5B%E2%98%83%20radio%201%5D%5D%22%2C%22images%22%3A%7B%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%22A%22%7D%2C%7B%22content%22%3A%22B%22%7D%2C%7B%22content%22%3A%22C%22%7D%5D%2C%22randomize%22%3Afalse%2C%22multipleSelect%22%3Afalse%2C%22displayCount%22%3Anull%2C%22noneOfTheAbove%22%3Atrue%2C%22onePerLine%22%3Atrue%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22hints%22%3A%5B%5D%7D)). Things to check:
    - All the usual functionality is preserved.
    - If you select multiple correct answers in the editor, one of which is replaced by 'None of the above', and select the remaining correct answers in the widget (but leave 'None of the above' unselected), the widget will be graded as correct (which is appropriate).

Reviewers: eater, jack

Reviewed By: jack

Differential Revision: http://phabricator.khanacademy.org/D9764