#217 Username, first name, and last name error handling.

Wrote error handling methods in the registration component to render the error div for the fields username, first name and last name. Had to make first name and last name were required fields by adding two validator methods to the CreateUserSerializer to make sure those two fields weren't empty when being posted to the server. Also wrote a corresponding integration test in test_user.py. Still need to error check for password and school fields. There was also a school_location reference in the registration component and since that no longer exists I changed it to school_international.
Why do we need all these functions? You can just have all this logic in `renderError` above:
```javascript
renderError: function(field) {
  // Notice that you can also access properties on objects with subscript notation, e.g. foo.bar == foo['bar']
  if (!this.state.errors[field]) {
    return null;
  }

  return (
    // JSX here...
  );
}
```