FSE blocks are missing additional CSS classes on the front

[carolinan]

Describe the bug
The FSE specific blocks does not output the CSS classes added in the Additional CSS option on the front.
(The CSS class saves fine in the editor.)

I have tested:
site title
post title
post content
post comments
post comments count
post comments form
post date
post excerpt
post featured image
post tags

I have not tested post author due to a PHP error.

To reproduce
Steps to reproduce the behavior:

1. Enable full site editing
2. Add any of the blocks listed above
3. Add a CSS class in the option in the block sidebar.
4. Save and view the source code of the front

Expected behavior
I expect the CSS class name to be available on the front.

Editor version (please complete the following information):

• WordPress version: 5.4.1
• Gutenberg: Master, latest commit was: b73bb6d

Desktop (please complete the following information):
Windows 10
Chrome version: 81.0.4044.129

[mtias]

This also reminds me we need to look at support for post_class and body_class in the appropriate wrappers.

[carolinan]

Closing this because it is a duplicate of #21903