Unify validation mandatory and valid messages into unique translations key

xe.admin.registration.fieldMandatory=This field is mandatory.
core.create.validation.mandatoryfield=Mandatory field
core.editors.validation.mandatoryField=This field is mandatory

Should be:
core.validation.required.message=This field is required.

and
xe.admin.registration.fieldOkay=Ok.
core.create.validation.valid=OK

Should be:
core.validation.valid.message=Ok.

According to http://platform.xwiki.org/xwiki/bin/view/DevGuide/VerticalForms 'Mandatory' should be renamed 'Required'