[Form] Client and server-side form validation

the core form components currently are doing validation only on the client side (e.g. checking "required" for a textfield). there is no server-side validation at all.

the forms helper logic of the AEM foundation components provide some support for server-side validation, it should be used to check the constraints the user can activate via the edit dialogs of the form components.

Start by reviewing the core form components and the AEM foundation components' forms helper logic mentioned in the issue. Determine how constraints enabled through the form components' edit dialogs are currently checked, then verify that the same constraints are validated server-side as well as client-side.

Written by the indexing model from the issue text.