Replace icon with details block

app/assets/javascripts/vue_shared/components/markdown/header.vue

@@ -234,7 +234,7 @@ export default {
             :prepend="true"
             tag-select="Click to expand"
             :button-title="__('Add a collapsible section')"
-            icon="chevron-right"
+            icon="details-block"
           />
           <toolbar-button
             :prepend="true"

app/views/shared/blob/_markdown_buttons.html.haml

@@ -16,7 +16,7 @@
                               data: { "md-tag" => "[{text}](url)", "md-select" => "url", "md-shortcuts": '["mod+k"]' },
                               title: sprintf(s_("MarkdownEditor|Add a link (%{modifier_key}K)") % { modifier_key: modifier_key }) })
 
-  = markdown_toolbar_button({ icon: "chevron-right", 
+  = markdown_toolbar_button({ icon: "details-block", 
                               data: { "md-tag" => "<details><summary>Click to expand</summary>\n{text}\n</details>", "md-prepend" => true, "md-select" => "Click to expand" },
                               title: _("Add a collapsible section") })