Fix HTML being rendered in mirror error message

In Rails 6, String manipulation methods are now HTML-safe
aware. https://github.com/rails/rails/pull/33990

This means, when doing the `#insert` here it first escapes
the HTML before inserting into the HTML-safe string

ee/app/helpers/ee/mirror_helper.rb

@@ -8,7 +8,7 @@ module EE
 
       return message if raw_message
 
-      message.insert(0, "#{icon('warning triangle')} ")
+      message = icon('warning triangle') + ' ' + message
 
       if can?(current_user, :admin_project, @project)
         link_to message, project_mirror_path(@project)

ee/spec/views/shared/_mirror_status.html.haml_spec.rb

@@ -42,6 +42,13 @@ describe 'shared/_mirror_status.html.haml' do
       expect(rendered).to have_content("The repository failed to update")
     end
 
+    it 'renders failure message with icon' do
+      render 'shared/mirror_status'
+
+      expect(rendered).to have_content('The repository failed to update')
+      expect(rendered).to have_css('i', class: 'fa-warning fa-triangle')
+    end
+
     context 'with a previous successful update' do
       it 'renders failure message' do
         @project.import_state.last_successful_update_at = Time.now - 1.minute