Added haml-lint and fix warnings (#2773)

* add haml_lint to Gemfile

* add .haml-lint.yml

* fix warnings of haml_lint

app/views/layouts/error.html.haml

@@ -1,11 +1,11 @@
 !!!
-%html{:lang => I18n.locale}
+%html{ lang: I18n.locale }
   %head
-    %meta{:content => "text/html; charset=UTF-8", "http-equiv" => "Content-Type"}/
-    %meta{:charset => "utf-8"}/
+    %meta{ content: 'text/html; charset=UTF-8', 'http-equiv' => 'Content-Type' }/
+    %meta{ charset: 'utf-8' }/
     %title= yield :page_title
-    %meta{:content => "width=device-width,initial-scale=1", :name => "viewport"}/
-    %link{:href => "https://fonts.googleapis.com/css?family=Roboto:400", :rel => "stylesheet"}/
+    %meta{ content: 'width=device-width,initial-scale=1', name: 'viewport' }/
+    %link{ href: 'https://fonts.googleapis.com/css?family=Roboto:400', rel: 'stylesheet' }/
     :css
       body {
         font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
@@ -31,6 +31,6 @@
       }
   %body
     .dialog
-      %img{:alt => "Mastodon", :src => "/oops.png"}/
+      %img{ alt: 'Mastodon', src: '/oops.png' }/
       %div
         %h1= yield :content