Node previews don't work when adding group node

The approach here is actually quite clean. If someone can port it to 2.x/3.x then I'd be willing to commit this. Not a fan that it's node-specific, but at least it bridges the gap between now and when I get around to solving the issue of extra entity form actions (other than save/cancel).

Patches for 2.x and 3.x. I've applied the 2.x patch on my dev site, but am only hopeful that the 3.x patch is functional.

There's a reroll for the changed `use` statements. Renamed 'group_content_enabler' to 'group_relation'. For 3.x updated the route to 'entity.group_relationship.create_form'.

Interdiff from #21 failed so it's instead a diff of the actual patch files.

Here is another approach:
- Instead of using hook_node_create_access, it is decorating the node preview access check service.
- Instead of checking the create permission, it is delectating the access check to the access control handler.
- Instead of using the hook_form_alter, it is using hook_form_BASE_FORM_ID_alter to alter the preview form.
This patch works with group 2 only. I will create another one for group 3 if the approach is acceptable.
@kristiaanvandeneynde please let me know thoughts?

Sorry, I forgot to add the decorator. Here is new patch.

I do like the decorator approach. Will have to find some time to go into detail, but could try and review this as part of my 3.3.0 release plan.