Ckozak/actkey overrides

The consumer_id column in cp_consumer_content_override had a not null constraint, which doesn't work anymore, because it should be null when (and only when) key_id is not null.  I don't think there's a way to have the database check that though.  I don't think that check should be necessary though.

The ContentOverrideResource class cannot use the @verify annotation because annotations cannot take variable parameters (the generic types in this case).  This is solved by requiring the implementing class (ConsumerContentOverrideResource for example) to define a findParentById method that looks up a Parent (Consumer in this case) using secure criteria, such that an exception is thrown when the parent is not found, which gives us the same result as the @verify annotation