add start date to sidebar and display dates on group and dashboard views when possible

app/views/shared/milestones/_sidebar.html.haml

@@ -17,7 +17,22 @@
       .value.hide-collapsed
         = milestone_progress_bar(milestone)
 
-    - if project
+    .block.start_date
+      .sidebar-collapsed-icon
+        = icon('calendar', 'aria-hidden': 'true')
+        %span.js-due-date-sidebar-value
+          = milestone.start_date.try(:to_s, :medium) || 'None'
+      .title.hide-collapsed
+        Start date
+        - if @project && can?(current_user, :admin_milestone, @project)
+          = link_to 'Edit', edit_namespace_project_milestone_path(@project.namespace, @project, @milestone), class: 'edit-link pull-right'
+      .value.hide-collapsed
+        %span.value-content
+          - if milestone.start_date
+            %span.bold= milestone.start_date.to_s(:medium)
+          - else
+            %span.no-value No start date
+
     .block.due_date
       .sidebar-collapsed-icon
         = icon('calendar', 'aria-hidden': 'true')
@@ -25,7 +40,7 @@
           = milestone.due_date.try(:to_s, :medium) || 'None'
       .title.hide-collapsed
         Due date
-          - if can?(current_user, :admin_milestone, @project)
+        - if @project && can?(current_user, :admin_milestone, @project)
           = link_to 'Edit', edit_namespace_project_milestone_path(@project.namespace, @project, @milestone), class: 'edit-link pull-right'
       .value.hide-collapsed
         %span.value-content