Migrate pfif-tools to Python 3

[nworden]

There's a standalone app (in tools/pfif-tools) that will also need to be migrated to Python 3.

Flask seems like a good choice to replace webapp2 for pfif-tools since the app's so simple. On second thought, I'll use Django, partially to be consistent with Person Finder itself, and partially because there's some hard-coded HTML printing in there and it'd be nice to be able to move that to Django templates. We can also treat it as a sort of test run of Django/Python 3 migration: this isn't blocked by any App Engine dependencies, so it'll be a chance to shake out some problems there earlier than we would otherwise.