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/api/v1/instances/show.rabl

@@ -1,6 +1,6 @@
 object false
 
-node(:uri)         { Rails.configuration.x.local_domain }
+node(:uri)         { site_hostname }
 node(:title)       { Setting.site_title }
 node(:description) { Setting.site_description }
 node(:email)       { Setting.site_contact_email }