Restrict creation of new content in the answer area

Summary:
There are three possible conditions:

Answer area is of type 'radio', 'table', 'input-number', or 'expression':
No change in editing flexibility.

Answer area is of type 'multiple':
Can no longer change type.

Answer area is of type 'multiple' and is empty:
Can no longer change type or add any content.

This diff keeps the `AnswerAreaEditor`'s child `Editor`/`WidgetEditor`
(i.e. hides it via css instead of not mounting it) because it is referenced
elsewhere in the file via `this.ref.editor`;

Test Plan:
No change in editing [this item](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22Hint%3A%20The%20answer%20is%2055%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22input-number%22%2C%22options%22%3A%7B%22value%22%3A%2255%22%2C%22simplify%22%3A%22enforced%22%2C%22size%22%3A%22normal%22%2C%22inexact%22%3Afalse%2C%22maxError%22%3A0.1%2C%22answerType%22%3A%22number%22%7D%2C%22calculator%22%3Afalse%7D%2C%22itemDataVersion%22%3A%7B%22major%22%3A0%2C%22minor%22%3A1%7D%2C%22hints%22%3A%5B%5D%7D)
No longer able to change answer area type in [this item](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22Blah%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%2242%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%5D%7D)
No longer able to edit answer area at all in [this item](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22Heh%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%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%22itemDataVersion%22%3A%7B%22major%22%3A0%2C%22minor%22%3A1%7D%2C%22hints%22%3A%5B%5D%7D)

Reviewers: justinjaffray, eater, aria

Reviewed By: eater, aria

Subscribers: eater, alpert

Differential Revision: http://phabricator.khanacademy.org/D6149