Add mandatory parent special case for amp-subscriptions

[westonruter]

See https://wordpress.org/support/topic/sanitizer-stripping-amp-subscriptions-encrypted-ciphertext/

The AMP spec uses the mandatory_parent key to refer to the tag name of the mandatory parent, except in one case for amp-subscriptions. This adds the special case.

[github-actions]

Plugin builds for d8ab291 are ready 🛎️!

• Download development build
• Download production build

Checksums

# Development build checksums
dc7cb46d9407fdc877a7d3c436105f2d27b38609ee78f36b8e5399e5ff694cb9 *amp.zip

# Production build checksums
f7844cb7f9d905ea79e4548e28dced1f9878e3d84c81bc54c0d1cf17115fa709 *amp.zip

Warning

These builds are for testing purposes only and should not be used in production.

[westonruter]

@thelovekesh We need this one included as well.