[Tails-l10n] (review)[de] wiki/src/doc/first_steps/startup_o…

Hi,
for version 1.7 of Tails I translated the offline-mode page.My branch
is based upon the "testing" branch, I hope that everything is as it
should be. (and I also dunno where these commits will be merged into).
Anyway, please review:

https://gitlab.com/spriver/tails.git
branch: de_offline_mode
commit: 49d0bbf Initial translation

Cheers!

spriver