Dashboard performance

This is a quick fix for #395 - it caches the attained_level displayed on the dashboard which greatly reduces the number of queries made to build the dashboard. (obviously this won't help the first display, but subsequent loads should be better).

It also contains:

* a development footer to show page generation stats (eg, db queries)
* a `cache:clear` rake task to help with clearing the cache in production

I've got some more in-depth fixes, though I'm working on them on another branch for now.