Add command to get discussion_link for a course

This will be used to detect courses that are linking to Stack Exchange
sites so we can handle them specially when generating data dumps.

lms/djangoapps/django_comment_client/management/commands/get_discussion_link.py

+from django.core.management.base import BaseCommand, CommandError
+
+from courseware.courses import get_course
+
+
+class Command(BaseCommand):
+    args = "<course_id>"
+
+    def handle(self, *args, **options):
+        if not args:
+            raise CommandError("Course id not specified")
+        if len(args) > 1:
+            raise CommandError("Only one course id may be specifiied")
+        course_id = args[0]
+
+        try:
+            course = get_course(course_id)
+        except ValueError:
+            raise CommandError("Invalid course id: {}".format(course_id))
+
+        if course.discussion_link:
+            self.stdout.write(course.discussion_link)