Added haml-lint and fix warnings (#2773)

* add haml_lint to Gemfile

* add .haml-lint.yml

* fix warnings of haml_lint

app/views/layouts/public.html.haml

@@ -7,9 +7,9 @@
     - if !user_signed_in? && single_user_mode?
       %span.single-user-login
         = link_to t('auth.login'), new_user_session_path
-        = "\u2014"
+        \u2014
     %span.domain= link_to site_hostname, root_path
     %span.powered-by
-      = t('generic.powered_by', link: link_to('Mastodon', 'https://github.com/tootsuite/mastodon')).html_safe
+      != t('generic.powered_by', link: link_to('Mastodon', 'https://github.com/tootsuite/mastodon'))
 
-= render template: "layouts/application"
+= render template: 'layouts/application'