archive description

There are a couple places in the code that require the repo description to be set. There is a default description for repo_description but not for archive_description. That led to a crash in fdroid nightly, if it starts trying to archive apps, since its generated config.py does not include archive_description. This sets repo_descriptionandarchive_descriptionin the generated _config.py_ and adds a new defaultarchive_description`. It also makes both defaults transatable.