enhance contribution counting accuracy

add some table joins to avoid counting dead contributions

issue #253
caching: stop storing keys

It removes a feature (template cache purging) but only for a minority
of users. However, it significantly decreases the amount of db writes
for *all* users and gets rid of some related issues.

fixes #521
tracking: don't count HEAD requests
add manual migration to delete HEAD requests

Since I did not log the request type, I delete by user agent: lwp, curl
and PritTorrent. These are not all HEAD requests, but most of them are.
improve HEAD-delete migration: free disk space
analytics: show release date more prominently
widgetize & prettify episode analytics
merge
improve tracking import

- purge DownloadIntentClean
- purge cache
- fill DownloadIntentClean after import
improve HEAD-delete migration

respect DownloadIntentClean table