Empathy: Add support for submitting answers to a prompy

Empathy: Add support for submitting answers to a prompy

For this we convert ActivePrompt from a function to a class-based
React component. Then we use the "uncontrolled components" technique
again to arrange for an array to hold the form values.

Finally, when answers are submitted successfully we display a simple,
static message. Eventually, we'll want to display some dynamic updates
as other players vote, (but for that, we'll also need the server to
give us that information).