Editor allows overflow-x:visible with exception for categorizer

Summary:
ToolTips were becoming impossible to read due to the editor's
overflow-x:scroll styling. I reset the editor to have overflow-x:visible
and gave categorizers overflow-x:scroll only when nested in the editor
interface. As far as I can tell, no other widgets are able to overflow in
the same manner, so it seems fine to make an exception for the categorizer.
Long-term, if we want the tooltips to overflow, then the editor will need
to have overflow-x:visible (i.e., there's no way around it).

Fixes T2591

Test Plan:
- See a long categorizer [here](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5Cn%5Cn%5B%5B%E2%98%83%20categorizer%201%5D%5D%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%22categorizer%201%22%3A%7B%22type%22%3A%22categorizer%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22items%22%3A%5B%22ababababababbababbabababbabababbabababbabababbabababa%22%5D%2C%22categories%22%3A%5B%22Dogs%22%2C%22Cats%22%5D%2C%22values%22%3A%5B%5D%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).
    - Notice that it has a scroll bar in the editor pane, but not the widget pane (i.e., on the left but not on the right).
    - Try some of the tooltips--notice that they overflow.
- For another example, see the sorter [here](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5Cn%5Cn%5B%5B%E2%98%83%20sorter%201%5D%5D%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%22sorter%201%22%3A%7B%22type%22%3A%22sorter%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22correct%22%3A%5B%22%24x%24%22%2C%22%24y%24%22%2C%22%24z%24%22%5D%2C%22layout%22%3A%22horizontal%22%2C%22padding%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). Again, the tooltips overflow.

Reviewers: jack

Reviewed By: jack

Subscribers: eater

Maniphest Tasks: T2591

Differential Revision: http://phabricator.khanacademy.org/D9513