show toot chars limit and poll limit in instance api (for mobile app)

app/serializers/rest/instance_serializer.rb

@@ -5,7 +5,8 @@ class REST::InstanceSerializer < ActiveModel::Serializer
 
   attributes :uri, :title, :short_description, :description, :email,
              :version, :urls, :stats, :thumbnail,
-             :languages, :registrations, :approval_required, :invites_enabled
+             :languages, :registrations, :approval_required, :invites_enabled,
+             :max_toot_chars, :poll_limits
 
   has_one :contact_account, serializer: REST::AccountSerializer
 
@@ -69,6 +70,19 @@ class REST::InstanceSerializer < ActiveModel::Serializer
     Setting.min_invite_role == 'user'
   end
 
+  def max_toot_chars
+    5000
+  end
+
+  def poll_limits
+    {
+      max_options: 10,
+      max_expiration: 2592000,
+      min_expiration: 300,
+      max_option_chars: 50
+    }
+  end
+
   private
 
   def instance_presenter