WIP issue-358 COPPA fix

Need to add validation and tidy things up. Also, maybe i should split all of this into a mixin?
Here is [the code](https://github.com/mozilla/fxa-content-server/commit/7b0ab452aecdf491368d56cae47d985bbf43b71c#diff-44670c71e73f70da9943027fedc5ef16R57) that I used before:
```js
    _canCreateAccount: function (month, day) {
      // Make the assumption that the user is 13 during this calendar year.
      // Find out whether they are 12 or 13 now.
      //
      // JS months go from 0-11. The months in the form go from 1-12 to be
      // comprehensible to humans.
      month = month - 1;
      var today = new Date();
      var todayMonth = today.getMonth();
      var todayDay = today.getDate();

      return ((month < todayMonth) ||
              (month === todayMonth && day <= todayDay));
```

This does not check the year, but should give us a basis to work from.