DateTime attribute: optimize and fix validation

* Let's load the attribute data only if needed (query atDateTimeSettings)
* Let's load the attribute data in validateForm: when akDateDisplayMode is date_time the method receives the keys 'value_dt', 'value_h', 'value_m', 'value_a' and not the key 'value'

Fixes bug http://www.concrete5.org/developers/bugs/5-6-2-1/adding-datetime-user-attribute-required-on-registration-form-blo/