Localise help links in Tails Greeter

Currently dealt with crudely by having the URL gettext:ised but we want to use the same code for generating localised URL:s as in WhisperBack.

The Greeter code lives in https://git-tails.immerda.ch/greeter. WhisperBack code lives in https://git-tails.immerda.ch/whisperback. It would be good to resolve #6452 first, but OTOH, having a second example use of the code to factorize can make refactoring easier, and more to the point.

Original created by @anonym on 6525 (Redmine)

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