Comments: Allow passing a custom class to the form container element in `comment_form()`.

Props cjbj, yahil, mayankmajeji, andraganescu, SergeyBiryukov.
Fixes #41009.
Built from https://develop.svn.wordpress.org/trunk@48393


git-svn-id: https://core.svn.wordpress.org/trunk@48162 1a063a9b-81f0-0310-95a4-ce76da25c4cd