Use Django's default methods to validate data on Media's fields

[bruvellu]

Django has built-in methods for validating the data of a model, like the handy Model.clean_fields(). We should use these to check if the metadata of 'Media' are being filled correctly.

See https://docs.djangoproject.com/en/dev/ref/models/instances/#validating-objects

Once this is implemented, we probably don't need the metadata_error field, for example:

https://github.com/bruvellu/cifonauta/blob/969b2b128648d480101c0063c163d8bfd36b8c10/meta/models.py#L83