Add support to enhance the cacheability of course assets.

This introduces a mechanism to control the time-to-live for an unlocked
course asset, which will allow browsers and intermediate proxies/caches
to cache these course assets, determinstically.

Locked assets, with their nature of requiring authorization, are not
eligible for caching.

cms/envs/common.py

@@ -306,6 +306,7 @@ simplefilter('ignore')
 
 MIDDLEWARE_CLASSES = (
     'request_cache.middleware.RequestCache',
+    'clean_headers.middleware.CleanHeadersMiddleware',
     'django.middleware.cache.UpdateCacheMiddleware',
     'django.middleware.common.CommonMiddleware',
     'django.middleware.csrf.CsrfViewMiddleware',
@@ -749,6 +750,7 @@ INSTALLED_APPS = (
 
     # For CMS
     'contentstore',
+    'contentserver',
     'course_creators',
     'external_auth',
     'student',  # misleading name due to sharing with lms