Validate hash using entity constraint instead of form validation

@krystalcode this is a really neat idea. +1 for moving validation out of forms and into constraints. I'm going to add some specific test coverage of constraint and check that we have and add if necessary coverage for the form.

I've open an MR and merged this patch to do this.

Added unit test and paired the patch back to the minimum necessary change.

I think this is a nice change that improves redirect validation and the ability to create redirects via an API.

Reviewed.

I've addressed @berdir's feedback - thanks for the review!

Looks good now, simpler and fewer unrelated changes.