Fix React warnings in InteractiveGraph

Summary:
Moves any methods called directly or indirectly by getEquationString
onto InteractiveGraph, so that getEquationString can be called by
InteractiveGraphEditor

Test Plan:
 * Add an interactive graph
 * For each interactive graph type:
    * change the answer to that type
    * move the answer
    * grade the question and check it grades as incorrect
    * move the answer to somewhere incorrect
    * grade the question and check it grades as incorrect
    * move the answer to the correct position
    * grade the question and check it grades as correct

Note: this fails on "circle", but circle appears to have been broken
before (it always grades as empty)

Reviewers: alex, alpert

Reviewed By: alex, alpert

Subscribers: brianmerlob, justinjaffray

Differential Revision: http://phabricator.khanacademy.org/D8201