Allow running mastodon on a different domain as the one used for identifying users (#1267)

* Allow running mastodon on a different domain as the one used for identifying users

* Alter documentation of WEB_DOMAIN to make clear it shouldn't be used unless the admin knows what they are doing

* Compare to web_domain instead of local_domain when dealing with feeds/API

* Correctly identify mentions to local accounts

Mentions URLs point to the person's web profile, i.e., the user page served on WEB_DOMAIN.

app/lib/tag_manager.rb

@@ -56,6 +56,10 @@ class TagManager
     id.start_with?("tag:#{Rails.configuration.x.local_domain}")
   end
 
+  def web_domain?(domain)
+    domain.nil? || domain.gsub(/[\/]/, '').casecmp(Rails.configuration.x.web_domain).zero?
+  end
+
   def local_domain?(domain)
     domain.nil? || domain.gsub(/[\/]/, '').casecmp(Rails.configuration.x.local_domain).zero?
   end