Install OnionCat

We should install OnionCat in Tails; it should probably not be an officially supported feature to start with, but having it available would ease the development of the VoIP support (#5709) feature.

It is in Debian Wheezy.

First, more testing is needed to ensure OnionCat is needed for whatever kind of VoIP support (#5709) we end up choosing. Let’s wait for that topic to mature a bit.

Related issues

Related to #5709

Original created by @tails on 5435 (Redmine)

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