Fix type field in thread list endpoint

The API's type field should come from the CS data's thread_type field,
not its type field, which distinguishes comments from threads.

lms/djangoapps/discussion_api/api.py

@@ -111,7 +111,6 @@ def _cc_thread_to_api_thread(thread, cc_user, staff_user_ids, ta_user_ids, group
             "group_id",
             "created_at",
             "updated_at",
-            "type",
             "title",
             "pinned",
             "closed",
@@ -127,6 +126,7 @@ def _cc_thread_to_api_thread(thread, cc_user, staff_user_ids, ta_user_ids, group
             "community_ta" if int(thread["user_id"]) in ta_user_ids else
             None
         ),
+        "type": thread["thread_type"],
         "raw_body": thread["body"],
         "following": thread["id"] in cc_user["subscribed_thread_ids"],
         "abuse_flagged": cc_user["id"] in thread["abuse_flaggers"],

lms/djangoapps/discussion_api/tests/test_api.py

@@ -369,7 +369,7 @@ class GetThreadListTest(CommentsServiceMockMixin, ModuleStoreTestCase):
             "anonymous_to_peers": False,
             "created_at": "1970-01-01T00:00:00Z",
             "updated_at": "1970-01-01T00:00:00Z",
-            "type": "discussion",
+            "thread_type": "discussion",
             "title": "dummy",
             "body": "dummy",
             "pinned": False,
@@ -421,7 +421,7 @@ class GetThreadListTest(CommentsServiceMockMixin, ModuleStoreTestCase):
                 "anonymous_to_peers": False,
                 "created_at": "2015-04-28T00:00:00Z",
                 "updated_at": "2015-04-28T11:11:11Z",
-                "type": "discussion",
+                "thread_type": "discussion",
                 "title": "Test Title",
                 "body": "Test body",
                 "pinned": False,
@@ -442,7 +442,7 @@ class GetThreadListTest(CommentsServiceMockMixin, ModuleStoreTestCase):
                 "anonymous_to_peers": False,
                 "created_at": "2015-04-28T22:22:22Z",
                 "updated_at": "2015-04-28T00:33:33Z",
-                "type": "question",
+                "thread_type": "question",
                 "title": "Another Test Title",
                 "body": "More content",
                 "pinned": False,
@@ -463,7 +463,7 @@ class GetThreadListTest(CommentsServiceMockMixin, ModuleStoreTestCase):
                 "anonymous_to_peers": False,
                 "created_at": "2015-04-28T00:44:44Z",
                 "updated_at": "2015-04-28T00:55:55Z",
-                "type": "discussion",
+                "thread_type": "discussion",
                 "title": "Yet Another Test Title",
                 "body": "Still more content",
                 "pinned": True,

lms/djangoapps/discussion_api/tests/test_views.py

@@ -154,7 +154,7 @@ class ThreadViewSetListTest(DiscussionAPIViewTestMixin, ModuleStoreTestCase):
             "anonymous_to_peers": False,
             "created_at": "2015-04-28T00:00:00Z",
             "updated_at": "2015-04-28T11:11:11Z",
-            "type": "discussion",
+            "thread_type": "discussion",
             "title": "Test Title",
             "body": "Test body",
             "pinned": False,