Feature: Add migrated, transferred checks

[kjohnson]

Description

This PR adds the give_is_form_migrated($formId) and give_is_form_donations_transferred($formId) functions for checking v3 references.

Testing Instructions

These functions can be used in the list table actions:

// \Give\DonationForms\V2\Endpoints\ListDonationForms::handleRequest

$migrate = $v2form
           && ! give_is_form_migrated($item['id']);

$transfer = $v2form
            && ! give_is_form_donations_transferred($item['id']);

[alaca]

Very simple and effective, I like it 👍