Combined Login/Registration Form

[ECOM-369](https://openedx.atlassian.net/browse/ECOM-369): Combine Registration & Login pages into single UI

Changes:
* Implement combined login and registration page.
* Add login/registration end-points to `user_api`
* Implement a minimal enrollment API to support the new login/registration page.

Notes:

* The old login and registration pages have not been changed.  We won't remove the old pages until after we complete an A/B test and send out a deprecation notice.

* To enable the new end-points, set the feature flag `ENABLE_COMBINED_LOGIN_REGISTRATION` to `True`.

* The new end-points are `/account/login` and `/account/register`

Reviewers: @dianakhuang @stephensanchez (feel free to tag other folks)

FYI: @rlucioni @AlasdairSwan 
Since this is a string, is it expecting the username as the ID? Or a string version of `User.id`?