categorizer: fix labels for mobile

Summary:
<label> elements don't work correctly on mobile safari, so here's a
pseudo-label implementation that should work on both web and mobile.

Test Plan:
tested on [localhost](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20categorizer%202%5D%5D%22%2C%22images%22%3A%7B%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fda8df81c78b22f5c69d477d8eabfb583968eaf84.png%22%3A%7B%22width%22%3A400%2C%22height%22%3A70%7D%2C%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fb59fc02ca1aae800977b8793ed22f647a1aa75ee.png%22%3A%7B%22width%22%3A425%2C%22height%22%3A150%7D%7D%2C%22widgets%22%3A%7B%22categorizer%202%22%3A%7B%22type%22%3A%22categorizer%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22items%22%3A%5B%221%22%2C%222%22%2C%223%22%5D%2C%22categories%22%3A%5B%22A%22%2C%22B%22%2C%22C%22%5D%2C%22values%22%3A%5B%5D%2C%22randomizeItems%22%3Afalse%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%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%22itemDataVersion%22%3A%7B%22major%22%3A0%2C%22minor%22%3A1%7D%2C%22hints%22%3A%5B%5D%7D)

and on mobile with the exercise "rules for multiplying and dividing
negative numbers"

Reviewers: michelle, eater

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D12700