Added instructions text to interactive graph widget.

Summary:
Added 'Click to add [points|vertices]' text to interactive graph
widget. Also required moving div.instructions from radio to general Perseus
stylesheet.

Test Plan:
- Create unlimited points and unlimited vertices widgets in Perseus.
- [Here](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5Cn%5B%5B%E2%98%83%20interactive-graph%201%5D%5D%5Cn%5Cn%5B%5B%E2%98%83%20interactive-graph%202%5D%5D%5Cn%5Cn%5B%5B%E2%98%83%20interactive-graph%203%5D%5D%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%22interactive-graph%201%22%3A%7B%22type%22%3A%22interactive-graph%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22step%22%3A%5B1%2C1%5D%2C%22backgroundImage%22%3A%7B%22url%22%3Anull%2C%22scale%22%3A1%2C%22bottom%22%3A0%2C%22left%22%3A0%7D%2C%22markings%22%3A%22graph%22%2C%22labels%22%3A%5B%22x%22%2C%22y%22%5D%2C%22showProtractor%22%3Afalse%2C%22showRuler%22%3Afalse%2C%22rulerLabel%22%3A%22%22%2C%22rulerTicks%22%3A10%2C%22range%22%3A%5B%5B-10%2C10%5D%2C%5B-10%2C10%5D%5D%2C%22gridStep%22%3A%5B1%2C1%5D%2C%22snapStep%22%3A%5B0.5%2C0.5%5D%2C%22graph%22%3A%7B%22type%22%3A%22point%22%2C%22numPoints%22%3A%22unlimited%22%7D%2C%22correct%22%3A%7B%22type%22%3A%22point%22%2C%22numPoints%22%3A%22unlimited%22%2C%22coords%22%3Anull%7D%7D%7D%2C%22interactive-graph%202%22%3A%7B%22type%22%3A%22interactive-graph%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22step%22%3A%5B1%2C1%5D%2C%22backgroundImage%22%3A%7B%22url%22%3Anull%2C%22scale%22%3A1%2C%22bottom%22%3A0%2C%22left%22%3A0%7D%2C%22markings%22%3A%22graph%22%2C%22labels%22%3A%5B%22x%22%2C%22y%22%5D%2C%22showProtractor%22%3Afalse%2C%22showRuler%22%3Afalse%2C%22rulerLabel%22%3A%22%22%2C%22rulerTicks%22%3A10%2C%22range%22%3A%5B%5B-10%2C10%5D%2C%5B-10%2C10%5D%5D%2C%22gridStep%22%3A%5B1%2C1%5D%2C%22snapStep%22%3A%5B0.5%2C0.5%5D%2C%22graph%22%3A%7B%22type%22%3A%22polygon%22%2C%22numSides%22%3A%22unlimited%22%2C%22snapTo%22%3A%22grid%22%7D%2C%22correct%22%3A%7B%22type%22%3A%22polygon%22%2C%22numSides%22%3A%22unlimited%22%2C%22coords%22%3Anull%2C%22snapTo%22%3A%22grid%22%7D%7D%7D%2C%22interactive-graph%203%22%3A%7B%22type%22%3A%22interactive-graph%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22step%22%3A%5B1%2C1%5D%2C%22backgroundImage%22%3A%7B%22url%22%3Anull%2C%22scale%22%3A1%2C%22bottom%22%3A0%2C%22left%22%3A0%7D%2C%22markings%22%3A%22graph%22%2C%22labels%22%3A%5B%22x%22%2C%22y%22%5D%2C%22showProtractor%22%3Afalse%2C%22showRuler%22%3Afalse%2C%22rulerLabel%22%3A%22%22%2C%22rulerTicks%22%3A10%2C%22range%22%3A%5B%5B-10%2C10%5D%2C%5B-10%2C10%5D%5D%2C%22gridStep%22%3A%5B1%2C1%5D%2C%22snapStep%22%3A%5B0.5%2C0.5%5D%2C%22graph%22%3A%7B%22type%22%3A%22linear%22%7D%2C%22correct%22%3A%7B%22type%22%3A%22linear%22%2C%22coords%22%3Anull%7D%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%22hints%22%3A%5B%5D%7D) is an example.

Reviewers: jack

Reviewed By: jack

Differential Revision: http://phabricator.khanacademy.org/D9225