Collect download statistics

The old site stores statistics in the database when a client requests for the appcast. This is a good way to track active users, and we should probably continue to do this. But since everything is static I have no idea how.

No files, tests, or entry points are named. Start by determining how the static Lektor site serves the appcast and where download requests could be recorded; done means defining and implementing a workable replacement for the old database-backed statistics while preserving static site delivery.

Written by the indexing model from the issue text.