Use django-storages's manifest files class

[humitos]

django-storages already implemented the manifest files class (see jschneier/django-storages#733 (comment)) so we can use it directly and remove our override for this in:

readthedocs.org/readthedocs/storage/s3_storage.py

Lines 55 to 61 in 933c316

	class S3StaticStorage(OverrideHostnameMixin, ManifestFilesMixin, S3Boto3Storage):
	"""
	An AWS S3 Storage backend for static media.
	* Uses Django's ManifestFilesMixin to have unique file paths (eg. core.a6f5e2c.css)
	"""