Implement working time graph for study group dashboard

(so far, without live update)

app/views/exercises/study_group_dashboard.html.slim

@@ -1,9 +1,25 @@
+- content_for :head do
+  // Force a full page reload, see https://github.com/turbolinks/turbolinks/issues/326.
+     Otherwise, code might not be highlighted correctly (race condition)
+  meta name='turbolinks-visit-control' content='reload'
+  = javascript_pack_tag('d3-tip', 'data-turbolinks-track': true)
+
 h1
   = t('.live_dashboard')
 
 div.teacher_dashboard data-exercise-id="#{@exercise.id}" data-study-group-id="#{@study_group_id}"
 
-h4
+h4.mt-4
+  = t('.time_spent_per_learner')
+
+.d-none#initial_graph_data data-graph_data=ActiveSupport::JSON.encode(@graph_data);
+div.w-100#chart_stacked
+
+.d-none.badge-info.container.py-2#no_chart_data
+  i class="fa fa-info" aria-hidden="true"
+  = t('.no_data_yet')
+
+h4.mt-4
   = t('.related_requests_for_comments')
 
 .table-responsive
@@ -17,5 +33,5 @@ h4
         th.col-12 = t('activerecord.attributes.request_for_comments.question')
         th = t('activerecord.attributes.request_for_comments.username')
         th.text-nowrap = t('activerecord.attributes.request_for_comments.requested_at')
-      tbody#posted_rfcs
-        = render(partial: 'request_for_comments/list_entry', collection: @request_for_comments, as: :request_for_comment)
+    tbody#posted_rfcs
+      = render(partial: 'request_for_comments/list_entry', collection: @request_for_comments, as: :request_for_comment)