add missing german translations necessary for backend

[hefan]

Contains necessary translations when using the backend in german. Otherwise the following errors occur on a fresh install:

app/views/spree/admin/orders/index.html.erb:153
translation missing: de.date.formats.default

app/views/spree/admin/shared/_js_locale_data.html.erb:9
undefined method `reject' for "translation missing: de.date.month_names":String

backend/vendor/assets/javascripts/solidus_admin/flatpickr/flatpickr.js:2167
TypeError: weekdays.join is not a function
(needs de.date.abbr_day_names)

/app/views/spree/admin/shared/_order_summary.html.erb:52
translation missing: de.time.formats.solidus.long

[tvdeyen]

Thanks. We should not maintain Rails translations in this file here.

[tvdeyen]

Danke :)