Instance helper to replace site title helper (#2038)

* Move site title helper to instance helper (name change only)

* Remove newline in <title> tag

* Add site_hostname helper method to wrap up local_domain value

* Use site_hostname helper in places that need local_domain value

app/views/about/show.html.haml

@@ -2,12 +2,12 @@
   = javascript_include_tag 'application_public', integrity: true
 
 - content_for :page_title do
-  = Rails.configuration.x.local_domain
+  = site_hostname
 
 - content_for :header_tags do
   %meta{ property: 'og:site_name', content: site_title }/
   %meta{ property: 'og:type', content: 'website' }/
-  %meta{ property: 'og:title', content: Rails.configuration.x.local_domain }/
+  %meta{ property: 'og:title', content: site_hostname }/
   %meta{ property: 'og:description', content: strip_tags(@instance_presenter.site_description.presence || t('about.about_mastodon')) }/
   %meta{ property: 'og:image', content: asset_url('mastodon_small.jpg') }/
   %meta{ property: 'og:image:width', content: '400' }/
@@ -72,7 +72,7 @@
           = t 'about.features.api'
 
   - unless @instance_presenter.site_description.blank?
-    %h3= t('about.description_headline', domain: Rails.configuration.x.local_domain)
+    %h3= t('about.description_headline', domain: site_hostname)
     %p= @instance_presenter.site_description.html_safe
 
   .actions