Make supsubs work correctly with character boxes

Summary:
In rule 18a. of the TeX book, it says we're supposed to set `u` and `v` to 0 if
the nucleus is a character box. Character boxes are loosely defined, but through
experimentation they are anything that contains a single character, including
ordgroups with a single element in them. This change makes KaTeX follow this
rule, and fixes T2404.

Test Plan:
- Make sure the new huxley screenshot looks good
- Make sure none of the huxley tests have changed

Reviewers: alpert

Reviewed By: alpert

Maniphest Tasks: T2404

Differential Revision: http://phabricator.khanacademy.org/D8275