Add icon when collapsed.

Hide and show icons in collapse and expanded mode.

app/assets/stylesheets/pages/issuable.scss

@@ -174,4 +174,26 @@
     background: $gray-normal;
     border: 1px solid $gray-darkest;
   }
+
+  &.right-sidebar-collapsed {
+    .issuable-count, 
+    .issuable-nav,
+    .assignee .title,
+    .assignee .selectbox,
+    .assignee .value .author,
+    .milestone,
+    .labels,
+    .participants,
+    .light,
+    .project-reference {
+      display: none;
+    }
+
+  }
+
+  &.right-sidebar-expanded {
+    .sidebar-collapsed-icon {
+      display: none;
+    }
+  }
 }

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

@@ -33,6 +33,8 @@
           = users_select_tag("#{issuable.class.table_name.singularize}[assignee_id]", placeholder: 'Select assignee', class: 'custom-form-control js-select2 js-assignee', selected: issuable.assignee_id, project: @target_project, null_user: true, current_user: true, first_user: true)
 
       .block.milestone
+        .sidebar-collapsed-icon
+          = icon('balance-scale')
         .title
           %label
             Milestone
@@ -54,7 +56,7 @@
           = f.submit class: 'btn hide'
 
       - if issuable.project.labels.any?
-        .block
+        .block.labels
           .title
             %label Labels
             - if can?(current_user, :"admin_#{issuable.to_ability_name}", @project)
@@ -87,7 +89,7 @@
               You're receiving notifications because you're subscribed to this thread.
 
       - project_ref = cross_project_reference(@project, issuable)
-      .block
+      .block.project-reference
         .title
         .cross-project-reference
           %span