Downgrade po4a to Stretch's 0.47-2

Originally created by @intrigeri on #17127 (Redmine)

It seems we won’t manage to do #17005 (closed) by upgrading All The Things to po4a 0.55 in time for Tails 4.0, so for now let’s do it by ensuring all relevant systems run 0.47-2.

Feature Branch: bugfix/17127-po4a-from-stretch

Related issues

Related to #17005 (closed)

Edited May 15, 2020 by intrigeri

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