Shift wiki and course.xml caching to use newly defined 'general' cache, separate from sessions left in 'default'

--HG--
branch : cache_separation

util/cache.py

@@ -0,0 +1,16 @@
+"""
+This module aims to give a little more fine-tuned control of caching and cache
+invalidation. Import these instead of django.core.cache.
+
+Note that 'default' is being preserved for user session caching, which we're 
+not migrating so as not to inconvenience users by logging them all out.
+"""
+from django.core import cache
+
+# If we can't find a 'general' CACHE defined in settings.py, we simply fall back
+# to returning the default cache. This will happen with dev machines.
+try:
+    cache = cache.get_cache('general')
+except ValueError:
+    cache = cache.cache
+