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
Several Changes to the License selector

* Added support for CC 4.0, CC0, Public Domain…
* Show Versions of CC with respect to their jurisdiction
Merge branch 'license_cc0and4' into release-1-10-22
Respect modification and commercial use parameters for CC 4.0 license preview
Merge branch 'license_cc0and4' into release-1-10-22
update readme
:shipit: release 1.10.22