Commit a531947 breaks ManifestFilesMixin storages

[TheEnbyperor]

Storage backends that use ManifestFilesMixin (e.g. S3ManifestStaticStorage) break on the new behaviour of exists() introduced in commit a531947.

The hashed_name function on the mixin first calls exists(), but this is now always False when overriding files is enabled, thus breaking the mixin.

[jschneier]

Dupe #1430