Multiple inputs in one form group not setting validity state correctly

[philippd]

If there are multiple valdr-validated inputs in one form group, the validity classes added on form group level (used to show and hide the validation messages via CSS) are not correct. The last validated input field sets the valdr-valid class and does not check if any other inputs in the same form group are still invalid at that time.

This issue is caused by the input directive in the valdr message module, which searches for it's parent form group element to add the validation message as it's last child and also set the validity states there.

A possible fix is to implement the form group as a new valdr directive, so we can conditionally require the form group from the valdr input directives and not have to do any "parent-dom-element-finding-magic" anymore.