add reapply_extensions instructor management command

lms/djangoapps/instructor/management/commands/reapply_extensions.py

+#!/usr/bin/python
+from django.http import Http404
+from django.core.management.base import BaseCommand, CommandError
+
+from courseware.courses import get_course_by_id
+from instructor.views.tools import fix_missing_extensions
+
+
+class Command(BaseCommand):
+    args = "<course_id>"
+    help = "Reapply all extensions (fixes extensions for newly added problems)"
+
+    def handle(self, *args, **options):
+        if len(args) != 1:
+            raise CommandError("insufficient arguments")
+        try:
+            course = get_course_by_id(args[0])
+            fix_missing_extensions(course)
+        except Http404 as e:
+            raise CommandError(e)