The purge command does not delete the transfer info

The history purge command does not delete the transfer info entries linked to the deleted history entries. This means that after the purge, these transfer info entries are left orphaned, referencing a transfer which no longer exists.

This causes problems with the 0.7.0 migration which adds a foreign key constraint to the transfer info table. If a purge was done at any time before the migration, the orphaned transfer info entries will fail the migration, because the foreign key constraint is violated. The migration should thus be adapted to take this possibility into account.

Note that, since the 0.7.0 version introduces a foreign key constraint on the transfer info table, the purge command should now work as intended, since the transfer info entries will now be deleted in cascade after the deletion of the history entries.