[cp mcr] Fix radio widget editing
[cp mcr] Fix radio widget editing

Summary:
Somehow radio widget editing has just been kinda broken for a while.

The first problem was that it was really hard to select correct choices in the
radio widget editor. This was because of some janky code to check whether you
were clicking on the actual editor itself of the area around the editors. I
cleaned that code up and made it more robust, so now you can easily choose
correct answers again.

The other problem was that, when selecting choices in a muliple-select radio
widget, it was impossible to un-select choices once they'd been selected!
Terrifying that that's been broken for a while! I fixed this by toggling the
state of the choice when it's clicked, instead of always setting it checked.

Test Plan:
 - Visit [this item](http://localhost:9000/#content=%7B%22question%22%3A%7B%22content%22%3A%22%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%22alignment%22%3A%22default%22%2C%22static%22%3Afalse%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22choices%22%3A%5B%7B%22content%22%3A%22wrong%22%7D%2C%7B%22content%22%3A%22right%22%7D%2C%7B%22isNoneOfTheAbove%22%3Afalse%2C%22content%22%3A%22wrong%22%7D%2C%7B%22isNoneOfTheAbove%22%3Afalse%2C%22content%22%3A%22right%22%7D%5D%2C%22randomize%22%3Afalse%2C%22multipleSelect%22%3Afalse%2C%22displayCount%22%3Anull%2C%22hasNoneOfTheAbove%22%3Afalse%2C%22deselectEnabled%22%3Afalse%7D%2C%22version%22%3A%7B%22major%22%3A1%2C%22minor%22%3A0%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22calculator%22%3Afalse%2C%22chi2Table%22%3Afalse%2C%22periodicTable%22%3Afalse%2C%22tTable%22%3Afalse%2C%22zTable%22%3Afalse%7D%2C%22itemDataVersion%22%3A%7B%22major%22%3A0%2C%22minor%22%3A1%7D%2C%22hints%22%3A%5B%5D%7D)
 - Open the radio widget editor
 - Choose some choices, notice that it's so easy to choose them
 - Turn on "multiple selections" and try choose and then un-choosing some of
   the choices. See that it works!

Reviewers: alex, johnsullivan

Reviewed By: johnsullivan

Subscribers: lauren, jared, #contentplatform

Differential Revision: https://phabricator.khanacademy.org/D35864

learningequality/perseus,ariabuckles/perseus,ariabuckles/perseus,learningequality/perseus,ariabuckles/perseus,ariabuckles/perseus