ConfigFormBase::buildForm() is missing a default submit handler

ConfigFormBase::buildForm() defines a submit button but does not attach a default submit handler to submit button which causes a problem with configuration forms not being saved if other modules attach their own submit handlers on top of the default ones.

Real world example: If a site has content_translation module enabled, then Account settings configuration form will not be saved because AccountSettingsForm::buildForm() method does not define a submit handler and content_translation_language_configuration_element_process() function, that wants to perform certain content translation actions, is attaching its own submit handler which becomes the only submit handler for the form.

ConfigFormBase::buildForm() should provide a default submit handler for the form so that submitForm() method is always called for configuration forms regardless of other submit handler additions made by other modules, unless specifically removed from the submit handlers list.