Add interactive-graph.js to Gruntfile

Test Plan:
After grunting loudly and copying perseus.min.js to the webapp repo, I
was able to insert an interactive graph widget.

Auditors: eater
Don't make widgets disappear when adding a hint

Test Plan:
In test.html, added a text input to the question area then added a hint.
The widget options didn't disappear!

Auditors: eater