Add upload_history_extractor

This is a Python 3 minor rewrite of munge_ddc.py; see also https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=966649

It adds a new file, upload_history_extractor.py, which prints output in the same format as the older munge_ddc.py. The existing upload_history_gatherer.py program should run peacefully on this thing's output, making it a drop-in replacement.

I'm hopeful to get feedback on this and merge it within 0-3 days. If people are +1, I'll fix cron jobs, etc., so that this code is live. That should restore service for the upload_history table.

Questions for reviewers:

• Is this file in the right place?

• Would you like more documentation?

• Is the code style OK?

I'd love to conform to UDD's expectations as much as possible. If you have questions that shouldn't block the merge, I'll answer those too.

I think that this has a bug where if two mbox files both contain the same message, we'll get duplicate data in UDD. I'll test for that problem before declaring victory. I'll probably fix it in the "gatherer" rather than here.

Long-term I think that there are other changes I'd make, but I may as well focus on restoring service first, then other improvements second.