FIX: Use Injector API for managing Member_Validator instance.

Updates the CMS profile page and SecurityAdmin to give developers a few ways to customise the required fields.

Added extension hook `updateValidator` for getValidator for things like modules to inject required fields to go along with Injector for replacing the entire class for project specific use.

Attached commits also clean up PHPDoc (though looking at the main commit https://github.com/wilr/sapphire/commit/a8031cd9ab0f716d67040b9d24e1c3dcb58369f1 is probably easier to review)