support multiple response types per client

The Dynamic Client Registration spec specifies multiple response_types
and grant_types per client
(https://openid.net/specs/openid-connect-registration-1_0.html#ClientMetadata).
Since grant_types can be inferred from response_types we should be able
to support both without needing to store grant_types.

This also helps with oidc-client-js which expects a client that supports
both "id_token" and "id_token token".

oidc_provider/models.py

@@ -30,6 +30,20 @@ JWT_ALGS = [
 ]
 
 
+class ResponseType(models.Model):
+    value = models.CharField(
+        max_length=30,
+        choices=RESPONSE_TYPE_CHOICES,
+        unique=True,
+        verbose_name=_(u'Response Type Value'))
+    description = models.CharField(
+        max_length=50,
+    )
+
+    def __str__(self):
+        return u'{0}'.format(self.description)
+
+
 class Client(models.Model):
 
     name = models.CharField(max_length=100, default='', verbose_name=_(u'Name'))
@@ -45,8 +59,7 @@ class Client(models.Model):
                     u' of their credentials. <b>Public</b> clients are incapable.'))
     client_id = models.CharField(max_length=255, unique=True, verbose_name=_(u'Client ID'))
     client_secret = models.CharField(max_length=255, blank=True, verbose_name=_(u'Client SECRET'))
-    response_type = models.CharField(
-        max_length=30, choices=RESPONSE_TYPE_CHOICES, verbose_name=_(u'Response Type'))
+    response_types = models.ManyToManyField(ResponseType)
     jwt_alg = models.CharField(
         max_length=10,
         choices=JWT_ALGS,
@@ -99,6 +112,12 @@ class Client(models.Model):
     def __unicode__(self):
         return self.__str__()
 
+    def response_type_values(self):
+        return (response_type.value for response_type in self.response_types.all())
+
+    def response_type_descriptions(self):
+        return [response_type.description for response_type in self.response_types.all()]
+
     @property
     def redirect_uris(self):
         return self._redirect_uris.splitlines()