fix(content-gates): resolve fatals and mismatched data types

[dkoo]

See discussion in #4251. As mentioned:

• Fixes several issues with the sanitize_gate method (UI has no expected description field, sanitize_access_rule converts an array of product IDs to strings via sanitize_text_field, etc)
• Changes the UI element for selecting subscription products to a FormTokenField: the UI only let you choose a single value, but the back-end expects an array of values.
• Corrects the content rule slug when evaluating the post_types rule (note plural)
• Fixes a fatal error due to mismatched parameters from the newspack_content_gate_restrict_post filter hook in class-content-gifting.php

[miguelpeixe]

Thank you for addressing these! Just left a minor comment.