Fix for hints being immutable in Perseus One.

This was a fun one to track down. It turns out that we were actually modifing
props directly in this instance, which is something that React discourages but
ignores, since all updates flow upwards and then come back down to overwrite
local props. The reason this didn't work in Perseus One is that the prop that
was modified was part of a Backbone.Model attribute, which meant that attribute
was already set to the new value by the time `EditorPage.onChange()` was
called, and `Model.set()` did not result in a "change" event that would
trigger `forceUpdate()` via the Backbone Mixin.

Test Plan:
Confirm that hints now update as expected when edited in Perseus One.

Auditors: joel