Replace apt-get by apt

where this is logical and useful. Don't do it in past news or things talking about Squeeze, nor infrastructural bits.

Otherwise mozilla/addon-sdk/source/python-lib/cuddlefish/_version.py
gets modified by `git archive', and then dpkg aborts due to unexpected
changes to the upstream source.

Debian's Vcs-Git is still called icedove.git.

The latest package we're built (1:45.8.0-3+tails2) was prepared from
tails/stretch, not from tails/jessie.

This reverts commit 25da5896.

This reverts commit 9cb48b8b.

This reverts commit fd215ee3