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. (The filter does strip all non digits before formatting it for display but those changes aren't reflected in the DB.) 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?