Code review comment for lp:~al-maisan/launchpad/unembargo-443075

This branch fixes bug #443075. When security fix packages are un-embargo'ed
they are copied to the primary archive. Since they become publicly available
now an announcement email needs to be sent out.

Please note: the branch at hand was tested on Soyuz dogfood to make sure
that all the tricky bits between the normal/restricted librarian work as
expected.

Pre-implementation call with Julian.

Tests to run:

    bin/test -t archive-views -t distroseriesqueue -t test_copypackage -t xx-archive -t test_packageupload -t soyuz.tests.test_publishing