Allow comments to be attached to any entity type

Comments can currently only be used with nodes, and they really should be able to be added to any entity type.

It seems like this could be done by making comments a field that is added to an entity or just providing an admin interface where you select which entities that comments should be able to be attached to. The latter might be easier to implement with the current architecture, and from a data perspective we'd just need to add an entity_type field to the comment table (and rename nid to entity_id).