Add AWS_INTEGRATION_ENABLED flag, instead of relying on AWS_VERIFICATION_BUCKET_NAME to determine whether AWS Cloud integration is visible to user

[tompollard]

In our env file, there is a variable called "AWS_VERIFICATION_BUCKET_NAME". The absence/presence of this variable is used to determine whether the site should display AWS related content on the site, via the following function:

physionet-build/physionet-django/user/awsverification.py

Lines 17 to 21 in a276764

	def aws_verification_available():
	"""
	Check whether the site is configured for AWS account authentication.
	"""
	return bool(settings.AWS_VERIFICATION_BUCKET_NAME)

It would be more intuitive (readable/explicit) to have a AWS_INTEGRATION_ENABLED flag. Probably the same applies for our Google Cloud integration.