Include missing ExtraMark translations

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

@@ -57,6 +57,8 @@
     },
     "extra_marks": {
       "menu_title": "Zusätzliche Markierungen",
+      "title": "Zusätzliche Markierung",
+      "title_plural": "Zusätzliche Markierungen",
       "create": "Markierung erstellen",
       "name": "Markierung",
       "short_name": "Abkürzung",

aleksis/apps/alsijil/frontend/messages/en.json

@@ -20,7 +20,9 @@
     },
     "extra_marks": {
       "menu_title": "Extra marks",
-      "create": "Create Extra Mark",
+      "title": "Extra mark",
+      "title_plural": "Extra marks",
+      "create": "Create Extra mark",
       "name": "Mark",
       "short_name": "Abbreviation",
       "colour_fg": "Foreground Colour",