feat: handle attributes, object and subject relation keys the same way

Removes the "reverse_" key prefix to mark relations as reversed and uses the same key format for attributes.

The reverse_ key prefix was not needed as the key already contained the direction of the relation (Subject__Relation__Object). If we combine the key with the currently edited entity, then we easily know if the relation is reversed or not.

Using the same key format for attributes unifies the way we describe relations, we simply always handle "keys" no matter the relation type, and we have a simple way to go from key to relation object.