Explicitly add 'type="button"' to button-group components

Summary: Some browsers assume 'type="submit"', which can cause funky behavior and generally
isn't what you want with a button-group like this.

Test Plan:
- Run docs; everything looks fine and button-groups work as before.
- Also tested in Perseus, where the button-groups for the Grapher were causing the question to unintentionally submit.

Auditors: joel