Keep better gettext string history when updating translations

This --previous flag should enable string diffs in Weblate where
applicable.
Remove python-format comment hints for gettext

My xgettext version uses a new flag python-brace-format for this now.
Magic line: git sed '/#xgettext:python-format$/d' horizons
Replace some instances of python-format with python-brace-format

And update documentation (those commens no longer are required).