[BB-1543] Enable prerequisite subsections feature

Created by: Agrendalath

This PR enables prereqeusite subsections feature

Testing instructions:

1. Checkout this branch on your Ocim devstack.
2. Configure Ocim with the .env used for development.
3. Run Ocim Shell and create a new instance:

   from instance.factories import production_instance_factory
   production_instance_factory(name='123', sub_domain='bb1576-test')

4. Run Ocim with make run.dev and spawn a new appserver. Check that the Configuration section has these two lines under EDXAPP_FEATURES:

   MILESTONES_APP: true
   ENABLE_PREREQUISITE_COURSES: true

Make sure you clean up the instance afterwards.