Some special entities can't be placed in Atlas

[historic_bruno]

Rubble entities have an active Decay component and because of how that's implemented, they can only be local entities. This condition is violated if a rubble entity is placed in Atlas or spawned with the units demo map, resulting in an error or crash.

One way to fix this would be to set the Decay component as inactive in the rubble template, and then add a method to Decay that allows changing the active status (to be used when the rubble entity is properly spawned). That way they could be placed or spawned as non-decaying, non-local entities without any errors.

It might also be nice to prevent some special entities from being placed in Atlas since they have no useful purpose.

[historic_bruno]

Also applies to the common siege templates introduced in r12937, which aren't intended to be spawned as entities.