Debian upstream packaging

[almet]

We've had numerous discussions in the past about how to integrate Dangerzone in Debian upstream, but it doesn't seem we have an issue about it, so here is one.

Here are the interesting resources on the matter:

• ITP (Intent to package) on the debian mailing list
• Discussion on the Tails issue tracker
• Package container images as Linux packages

The main blocker for inclusion upstream seemed to be the huge package size (because it included the container image), but this will potentially go away with #1006.

We want to package Dangerzone for Debian, probably aiming for contrib rather than main, because of the way we ship and update container images (see #1067 for more context).