I found this while executing some AutomaticWebTest on dependent cubes, but this case can occur anyway.

rssitem call rssitem view recursively following "comments" relation, so we may
enter in a loop here.

Fix this by using a "seen" set() which is passed resursively and we skip
already processed entities.
Add a basic test using a comment commenting a comment commenting the first one.

Add a basic test to cover the comment form rendering.

The declaration contains a typo "__permissions__x" instead of "__permissions__" since 0cd78b11b571
Drop the whole declaration since this is the same permissions as
EntityType.__permissions__ (default permissions), so we don't need any migration.