Change belongs_to_required_by_default to true (#5888)

config/application.rb

@@ -18,6 +18,9 @@ require_relative '../lib/mastodon/redis_config'
 
 module Mastodon
   class Application < Rails::Application
+    # Initialize configuration defaults for originally generated Rails version.
+    config.load_defaults 5.1
+
     # Settings in config/environments/* take precedence over those specified here.
     # Application configuration should go into files in config/initializers
     # -- all .rb files in that directory are automatically loaded.