Make ID_KEY configurable (again)

[nijel]

Expected behaviour

ID_KEY should be configurable as that is needed by at least some services.

Actual behaviour

It is currently hardcoded for each backend.

Any other comments?

Seznam backend currently allows to configure this:

social-core/social_core/backends/seznam.py

Lines 29 to 30 in 0925304

	def get_user_id(self, details, response):
	return response.get(self.setting("ID_KEY") or self.ID_KEY)

But it really doesn't work because of:

social-core/social_core/utils.py

Lines 192 to 200 in b9d9937

	# Normally when resuming a pipeline, request_data will be empty. We
	# only need to check for a uid match if new data was provided (i.e.
	# if current request specifies the ID_KEY).
	if backend.ID_KEY in request_data:
	id_from_partial = partial.kwargs.get("uid")
	id_from_request = request_data.get(backend.ID_KEY)
	if id_from_partial != id_from_request:
	partial_matches_request = False

See #854 for more info.