Add a slider based on submissions

app/views/exercises/external_users/statistics.html.slim

@@ -1,11 +1,22 @@
 h1 = "#{@exercise} (external user #{@external_user})"
+- submissions = Submission.where("user_id = ?  AND exercise_id = ?", @external_user.id, @exercise.id).to_a
+- current_index = submissions.length - 1
+- current_submission = submissions[current_index]
+- files = current_submission.files.to_a
 
 #editor.row data-exercise-id=@exercise.id data-message-depleted=t('exercises.editor.depleted') data-user-id=@external_user.id
-  div class=(@exercise.hide_file_tree ? 'hidden col-sm-3' : 'col-sm-3') id='files' data-entries=FileTree.new(@exercise.files).to_js_tree
+  div class=(@exercise.hide_file_tree ? 'hidden col-sm-3' : 'col-sm-3') id='files' data-entries=FileTree.new(files).to_js_tree
   div id='frames' class=(@exercise.hide_file_tree ? 'col-sm-12' : 'col-sm-9')
-    - @exercise.files.each do |file|
+    - files.each do |file|
       = render('editor_frame', exercise: @exercise, file: file)
 
 #slider
+    input type='range' orient='horizontal' list='datapoints' min=0 max=submissions.length-1 value=current_index
+    datalist#datapoints
+        - index=0
+        - submissions.each do |submission|
+            option data-submission=submission
+                =index
+            - index += 1
 
 #timeline