add total number of submissions for exercise collections

app/views/exercise_collections/statistics.html.slim

@@ -3,6 +3,7 @@ h1 = @exercise_collection
 = row(label: 'exercise_collections.name', value: @exercise_collection.name)
 = row(label: 'exercise_collections.updated_at', value: @exercise_collection.updated_at)
 = row(label: 'exercise_collections.exercises', value: @exercise_collection.exercises.count)
+= row(label: 'exercise_collections.submissions', value: @exercise_collection.exercises.joins(:submissions).count)
 = row(label: 'exercises.statistics.average_worktime', value: @exercise_collection.average_working_time.round(3).to_s + 's')
 
 #graph

config/locales/de.yml

@@ -137,6 +137,7 @@ de:
         use_anomaly_detection: "Abweichungen in der Arbeitszeit erkennen"
         updated_at: "Letzte Änderung"
         exercises: "Aufgaben"
+        submissions: "Gesamtanzahl Submissions"
       user_exercise_feedback:
         user: "Autor"
         exercise: "Aufgabe"

config/locales/en.yml

@@ -137,6 +137,7 @@ en:
         use_anomaly_detection: "Enable Worktime Anomaly Detection"
         updated_at: "Last Update"
         exercises: "Exercises"
+        submissions: "Submissions (accumulated)"
       user_exercise_feedback:
         user: "Author"
         exercise: "Exercise"