MA-95: Update Registration endpoint to support 3rd party credentials

Automatically link edX account with social account in login mobile API path

lms/urls.py

@@ -595,6 +595,8 @@ if settings.FEATURES.get('ENABLE_THIRD_PARTY_AUTH'):
             oauth_exchange.views.AccessTokenExchangeView.as_view(),
             name="exchange_access_token"
         ),
+        # NOTE: The following login_oauth_token endpoint is DEPRECATED.
+        # Please use the exchange_access_token endpoint instead.
         url(r'^login_oauth_token/(?P<backend>[^/]+)/$', 'student.views.login_oauth_token'),
     )