add max_toot_chars from glitch-soc

app/serializers/rest/instance_serializer.rb

@@ -4,7 +4,7 @@ class REST::InstanceSerializer < ActiveModel::Serializer
   include RoutingHelper
 
   attributes :uri, :title, :short_description, :description, :email,
-             :version, :urls, :stats, :thumbnail,
+             :version, :urls, :stats, :thumbnail, :max_toot_chars,
              :languages, :registrations, :approval_required
 
   has_one :contact_account, serializer: REST::AccountSerializer
@@ -39,6 +39,10 @@ class REST::InstanceSerializer < ActiveModel::Serializer
     instance_presenter.thumbnail ? full_asset_url(instance_presenter.thumbnail.file.url) : full_pack_url('media/images/preview.jpg')
   end
 
+  def max_toot_chars
+    StatusLengthValidator::MAX_CHARS
+  end
+
   def stats
     {
       user_count: instance_presenter.user_count,