One mistake I never thought I'd make!

Summary:
I broke widgets because previously our markdown caching tricks
required that we never set reuseMarkdown to true during a render
where any old widget has changed props.

This broke because I started setting reuseMarkdown to false
whenever highlightedWidgets changed, and the editor changes
highlightedWidgets on every editor change (ugh).

I removed that if statement and requirement, since we probably
want our widgets to update props whenever they get new props.

Test Plan:
 * Open [this item](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20image%201%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%22image%201%22%3A%7B%22type%22%3A%22image%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22range%22%3A%5B%5B0%2C10%5D%2C%5B0%2C10%5D%5D%2C%22box%22%3A%5B620%2C400%5D%2C%22backgroundImage%22%3A%7B%22url%22%3A%22http%3A%2F%2Fstar1025.com%2Fwp-content%2Fblogs.dir%2F23%2Ffiles%2F2013%2F01%2Ftswift-620x400.jpg%22%2C%22width%22%3A620%2C%22height%22%3A400%7D%2C%22labels%22%3A%5B%7B%22content%22%3A%22hello%22%2C%22coordinates%22%3A%5B0%2C0%5D%2C%22alignment%22%3A%22below%22%7D%5D%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)
   (courtesy @alex) and change the label alignment.
 * verify that the label moved

Auditors: alex, michelle