Enabled `Live Preview` on WPORG

[jeffpaul]

A blueprint exists https://github.com/WordPress/plugin-check/blob/trunk/.wordpress-org/blueprints/blueprint.json so please have whoever is a committer for the WPORG plugin enable the Live Preview option so folks can test PCP on the WPORG plugin page (as I just tried to do with one of our plugins until I saw the Live Preview didn't exist).

[davidperezgar]

I'm going to check. Thanks Jeff.