Improve definitions of the custom completion rules in the standard modules

Something that we should do and recommend for other developers:
1. checkbox should be converted to advcheckbox to avoid repeated if (empty(...))
2. validation can be moved into the definition so the function add_completion_rules() from module forms can be easier reused for bulk completion, there are methods in moodleform for it: $this->_form->addRule() and $this->_form->addFormRule()