REST API: Disable anonymous commenting by default.

Adding a brand new anonymous comment method is a potential conduit for spam. Since it's still useful functionality, we're now hiding it behind a filter to allow plugins and themes to turn it on if they do want it.

Props helen, rachelbaker, joehoyle.
Fixes #38855.

Built from https://develop.svn.wordpress.org/trunk@39327


git-svn-id: https://core.svn.wordpress.org/trunk@39267 1a063a9b-81f0-0310-95a4-ce76da25c4cd