Change belongs_to_required_by_default to true (#5888)

app/models/account.rb

app/models/account_domain_block.rb

app/models/admin/action_log.rb

app/models/block.rb

app/models/conversation_mute.rb

app/models/favourite.rb

app/models/follow.rb

app/models/follow_request.rb

app/models/import.rb

app/models/invite.rb

app/models/list.rb

app/models/list_account.rb

app/models/media_attachment.rb

app/models/mention.rb

app/models/mute.rb

app/models/notification.rb

app/models/report.rb

app/models/session_activation.rb

app/models/status.rb

app/models/status_pin.rb