Add registration extra fields setting to configuration.

This exposes a Django setting to configure the registration form.  The values in this PR are identical to the [current defaults](https://github.com/edx/edx-platform/blob/master/lms/envs/common.py#L1425-L1441)

Eventually, we'll need to override the defaults to set `country` to "optional" for [ECOM-128](https://openedx.atlassian.net/browse/ECOM-128).

@jarv please review.