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.

I ran into this error. The patch in #26 wouldn't apply to Group 3.3.0, but the patch in #24 applied and resolved the error. I'm tempted to mark this as RTBC, but it sounds like the intention is to follow the decorator approach, which IMHO currently needs work.

Is there a version available for group 2.3?

Patch for group 2.3.x (tested using 2.3.2).