Remove use of non-relational fields in select_related queries

These changes should not change the actual queries at all, and those
will throw errors in Django 1.11.

openedx/core/djangoapps/user_api/course_tag/api.py

@@ -36,7 +36,7 @@ class BulkCourseTags(object):
                 and the secondary key is the course tag's key
         """
         course_tags = defaultdict(dict)
-        for tag in UserCourseTag.objects.filter(user__in=users, course_id=course_id).select_related('user__id'):
+        for tag in UserCourseTag.objects.filter(user__in=users, course_id=course_id).select_related('user'):
             course_tags[tag.user.id][tag.key] = tag.value
         get_cache(cls.CACHE_NAMESPACE)[cls._cache_key(course_id)] = course_tags