Refactor storage

[stsewd]

Currently:

• We have mix of file system storage with cloud storage methods mixed together
• Not all storage backends support the same features. So, we can't use the methods from build media on other storage backends.
• We are subclassing just to override some values

So, I'm refactoring our storage classes to:

• Have a common interface, this interface needs to be implemented for local and cloud storage classes.
• All storage classes share this same interface, so we can use the same methods in all of them.

Some other changes:

• copy_directory/walk was no longer used, so it was deleted (it was replaced by rclone sync).
• get_available_name on the cloud storage class doesn't need overriding, as the storage backend now allows overriding files (true by default).
• get_available_name on the file system storage class doesn't need overriding, as the storage backend now supports overriding files (false by default, but it's passed as an extra option in STORAGES).

This is just the start to make sure nothing breaks. After this I want:

• Move more options to be declared in STORAGES instead of subclassing.
• Make our tests rely less on the local storage classes
• Re-implement delete_directory to make use of the delete prefix S3 API. Currently, we are deleting file by file, which is slow, and generates lots of API calls.