Add validate method back to ListSerializer

[LilyFirefly]

Fixes #2168.

[LilyFirefly]

This is a large copy/paste from Serializer.run_validation. I would prefer to refactor this to be more DRY, but I'm not sure how best to do this.

[lovelydinosaur]

If we dropped the ValidationError and DjangoValidationError handling code into a function that takes an exception and returns the details for ValidationError to be raised, we could have that just in once place, which would be better.

[lovelydinosaur]

We could also consider pushing this... https://github.com/tomchristie/django-rest-framework/blob/master/rest_framework/fields.py#L307-L320 into a validate_empty_vaues

[lovelydinosaur]

I would probably recommend we drop the call to super here, in the same way we do for Serializer. It gets pretty confusing otherwise. This'd also let us move the if not isinstance(data, list) check out of to_internal_value which would be nicer and would mirror the Serializer classes if not isinstance(data, dict) check that occurs.

[lovelydinosaur]

Going to take a look at some of this now based on your input.