actually make comment section 3.10 compatible: this div is now handled by the layout, no need for match_kwargs, no more 'Component' added to domid

avoid empty div when no comments and no right to add comment. Remove spurious parameters from ajax place holder url.

cleanup/refactor/document comment form generation on the way

fix comment's form handling: don't put comments relation in hidden section, simply use an HiddenInput. Also fix new comment detection

use standard form validation instead of custom json controller's methods, support to apply the commment section component on entity not yet created (will be created with the comment)