Add missing translation

aleksis/apps/alsijil/frontend/components/Coursebook.vue

 <template>
   <c-r-u-d-iterator
-      i18n-key="coursebook"
+      i18n-key="alsijil.coursebook"
       :gql-query="gqlQuery"
       :gql-additional-query-args="gqlQueryArgs"
       :enable-create="false"

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

@@ -35,6 +35,7 @@
     "coursebook": {
       "menu_title": "Kursbuch",
       "page_title": "Kursbuch für {name}",
+      "title_plural": "Kursbuch",
       "summary": {
         "topic": "Thema",
         "homework": {

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

@@ -35,6 +35,7 @@
     "coursebook": {
       "menu_title": "Coursebook",
       "page_title": "Coursebook for {name}",
+      "title_plural": "Coursebook",
       "summary": {
         "topic": "Topic",
         "homework": {