Add avocado dependency to metadata migration

Alter the data migration template that produces metadata fixture loading
data migrations (triggered with the 'migration' management command) to
calculate the latest avocado migration at the time and include it as a
dependency of the metadata migration. This solves the problem of avocado
metadata being loaded into older versions of the avocado data structure
than they were generated from. Particularly, this fixes #185 where
metadata migrations were being executed before any avocado migrations
had occured on empty databases (new harvest installs with imported
metadata).