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.

common/djangoapps/clean_headers/middleware.py

@@ -0,0 +1,25 @@
+"""
+Middleware used for cleaning headers from a response before it is sent to the end user.
+"""
+
+
+class CleanHeadersMiddleware(object):
+    """
+    Middleware that can drop headers present in a response.
+
+    This can be used, for example, to remove headers i.e. drop any Vary headers to improve cache performance.
+    """
+
+    def process_response(self, _request, response):
+        """
+        Processes the given response, potentially stripping out any unwanted headers.
+        """
+
+        if len(getattr(response, 'clean_headers', [])) > 0:
+            for header in response.clean_headers:
+                try:
+                    del response[header]
+                except KeyError:
+                    pass
+
+        return response