Migrates the alert on the new branch page

Moves the alert to use gitlab UI classes

app/views/projects/branches/new.html.haml

@@ -2,8 +2,10 @@
 - default_ref = params[:ref] || @project.default_branch
 
 - if @error
-  .alert.alert-danger
-    %button.close{ type: "button", "data-dismiss" => "alert" } ×
+  .gl-alert.gl-alert-danger
+    = sprite_icon('error', size: 16, css_class: 'gl-icon gl-alert-icon gl-alert-icon-no-title')
+    %button.js-close.gl-alert-dismiss{ type: 'button', 'aria-label' => _('Dismiss') }
+      = sprite_icon('close', size: 16, css_class: 'gl-icon')
     = @error
 %h3.page-title
   New Branch