Add German translations for frontend

aleksis/apps/resint/frontend/messages/de.json

+{
+  "resint": {
+    "menu_title": "Dokumente",
+    "manage_posters": {
+      "menu_title": "Dokumente verwalten"
+    },
+    "poster_groups": {
+      "menu_title": "Dokumentengruppen"
+    },
+    "live_documents": {
+      "menu_title": "Live-Dokumente"
+    }
+  }
+}