Ng control group validators

[vsavkin]

Currently, it is not possible to add validators to control groups in the template. This limits template driven forms. This PR makes it possible.

<form some-form-validator>
  <div same-passwords>
    <input ng-control="password">
    <input ng-control="passwordConfirm">
  </div>
</form>

[mary-poppins]

Merging PR #4954 on behalf of @mhevery to branch presubmit-mhevery-pr-4954.

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