Configure LMS to select oauth2 providing library.

Available backends:

* django-oauth-toolkit (DOT)
* django-oauth2-provider (DOP)

* Use provided client ID to select backend for
  * AccessToken requests
  * third party auth-token exchange
* Create adapters to isolate library-dependent functionality
* Handle django-oauth-toolkit tokens in edX DRF authenticator class

MA-1998
MA-2000

lms/urls.py

@@ -820,10 +820,19 @@ if settings.FEATURES.get('AUTH_USE_OPENID_PROVIDER'):
 
 if settings.FEATURES.get('ENABLE_OAUTH2_PROVIDER'):
     urlpatterns += (
+        # These URLs dispatch to django-oauth-toolkit or django-oauth2-provider as appropriate.
+        # Developers should use these routes, to maintain compatibility for existing client code
+        url(r'^oauth2/', include('lms.djangoapps.oauth_dispatch.urls')),
+        # These URLs contain the django-oauth2-provider default behavior.  It exists to provide
+        # URLs for django-oauth2-provider to call using reverse() with the oauth2 namespace, and
+        # also to maintain support for views that have not yet been wrapped in dispatch views.
         url(r'^oauth2/', include('edx_oauth2_provider.urls', namespace='oauth2')),
+        # The /_o/ prefix exists to provide a target for code in django-oauth-toolkit that
+        # uses reverse() with the 'oauth2_provider' namespace.  Developers should not access these
+        # views directly, but should rather use the wrapped views at /oauth2/
+        url(r'^_o/', include('oauth2_provider.urls', namespace='oauth2_provider')),
     )
 
-
 if settings.FEATURES.get('ENABLE_LMS_MIGRATION'):
     urlpatterns += (
         url(r'^migrate/modules$', 'lms_migration.migrate.manage_modulestores'),
@@ -888,14 +897,6 @@ if settings.FEATURES.get('ENABLE_THIRD_PARTY_AUTH'):
 
 # OAuth token exchange
 if settings.FEATURES.get('ENABLE_OAUTH2_PROVIDER'):
-    if settings.FEATURES.get('ENABLE_THIRD_PARTY_AUTH'):
-        urlpatterns += (
-            url(
-                r'^oauth2/exchange_access_token/(?P<backend>[^/]+)/$',
-                auth_exchange.views.AccessTokenExchangeView.as_view(),
-                name="exchange_access_token"
-            ),
-        )
     urlpatterns += (
         url(
             r'^oauth2/login/$',