EVENT-159: Allow international phone numbers to be valid inputs

Doesn't strip all non digit characters before the answer value is persisted to the API yet. Where would be the best place to do this? If I do it in `validateRegistrant.js`, it messes with the user's input and immediately removes characters and puts the cursor at the end. Would this functionality belong in the `AnswerCache.js`? Or should this be done server-side?
Where should validation be done? As the user progresses through the form or at the very end? The email question is validated before allowing the user to continue through the form. I added this here to validate before continuing but I could remove it.