Fix IDN mentions not being processed, IDN domains not being rendered (#12715)

This changes the REST API to return unicode domains in the `acct`
attribute instead of punycode, and to render unicode instead of
punycode on public HTML pages as well.

Fix #7812, fix #12246

app/models/account.rb

@@ -50,7 +50,7 @@
 
 class Account < ApplicationRecord
   USERNAME_RE = /[a-z0-9_]+([a-z0-9_\.-]+[a-z0-9_]+)?/i
-  MENTION_RE  = /(?<=^|[^\/[:word:]])@((#{USERNAME_RE})(?:@[a-z0-9\.\-]+[a-z0-9]+)?)/i
+  MENTION_RE  = /(?<=^|[^\/[:word:]])@((#{USERNAME_RE})(?:@[[:word:]\.\-]+[a-z0-9]+)?)/i
 
   include AccountAssociations
   include AccountAvatar
@@ -164,6 +164,10 @@ class Account < ApplicationRecord
     local? ? username : "#{username}@#{domain}"
   end
 
+  def pretty_acct
+    local? ? username : "#{username}@#{Addressable::IDNA.to_unicode(domain)}"
+  end
+
   def local_username_and_domain
     "#{username}@#{Rails.configuration.x.local_domain}"
   end