Add a bundle subclass for forum.module's business logic

Forum is approved for removal. See #1898812: [policy] Deprecate forum module for removal in Drupal 11 →

This is now Postponed. The status is set according to two policies. The Remove a core extension and move it to a contributed project → and the Extensions approved for removal → policies.

It will be moved to the contributed extension → once the Drupal 11 branch is open.

FWIW, one thing to consider is backwards compatibility. If a custom site also defined a bundle class for the forum node type, then either this won't be used as it's overwritten, resulting in fatal errors/non-executed blocks of code if they check for an interface or so. Or the custom class is replaced, equally resulting in either fatal errors or code not being executed.

BC would be quite challenging to handle.

#8: Bundle classes are just a neat way to organize code, they don't allow to do something that can't be done otherwise as well. In this case, a setting to configure which node types could be used in forum. The only difference is where that setting is checked exactly, in node hook or so, or in the bundle info alter hook. In other words, not really related.

Yeah we needed to do this before folks had a chance to write their own

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