When loading a chain that had been fully serialized to json, deserialization
happened only in part of the original task. Specifically, take a
json-serialized task that looks like the following:

    {
        ...,
        "kwargs": {
            "tasks": [
                {...}, {...}, {...}
            ]
        }
    }

After calling `celery.signature(that_thing_above)`, we get an object that
actually looks like this:

    {
        ...,
        "kwargs": {
            "tasks": [
                task_1, # an instance of celery.Signature
                {...}, {...} # same as before deserialization
            ]
        }
    }

The culprit was `chain.from_dict`, which was converting only the first subtask
of the chain to actual `Signature` instances. This commit changes that
behaviour and converts all subtasks to signatures instead. Without this, some
operations on the chain object, such as calling `chain.on_error(...)` would
cause errors of the form `'dict' object has no attribute 'xyz'`.