Added haml-lint and fix warnings (#2773)

* add haml_lint to Gemfile

* add .haml-lint.yml

* fix warnings of haml_lint

app/views/layouts/embedded.html.haml

@@ -1,7 +1,7 @@
 !!! 5
-%html{:lang => I18n.locale}
+%html{ lang: I18n.locale }
   %head
-    %meta{:charset => 'utf-8'}/
+    %meta{ charset: 'utf-8' }/
     = stylesheet_link_tag 'application', media: 'all'
     = javascript_pack_tag 'public', integrity: true, crossorigin: 'anonymous'
   %body.embed