First round of changes for #875

- $validate property is now $validation_rules
- $insert_validate_rules is now $insert_validation_rules
- Separated retrieval of validation rules
($this->get_validation_rules('update')) from validation
($this->validate($data, 'update'), with $this->validate() calling
$this->get_validation_rules()), which also addresses the program flow

- Updated documentation accordingly
Merge branch 'develop' of https://github.com/ci-bonfire/Bonfire into develop