Read from doc url mapping file at load time, rather than once per request

cms/djangoapps/contentstore/context_processors.py

 import ConfigParser
 from django.conf import settings
 
+config_file = open(settings.REPO_ROOT / "docs" / "config.ini")
+config = ConfigParser.ConfigParser()
+config.readfp(config_file)
 
-def doc_url(request):
-    config_file = open(settings.REPO_ROOT / "docs" / "config.ini")
-    config = ConfigParser.ConfigParser()
-    config.readfp(config_file)
 
+def doc_url(request):
     # in the future, we will detect the locale; for now, we will
     # hardcode en_us, since we only have English documentation
     locale = "en_us"