* Add Rclone storage backend
* Support templating the name of files stored via Rclone
* Enable Rclone accounting
* Remove redundant type conversion
* Provide progress information for Rclone download/upload operations
* Log error when Rclone can't instantiate filesystem
* Remove filename templating functionality for Rclone storage
To (maybe) be later reintroduced as a generic functionality for all
storage backends.
* Remove S3 specific check
* Move Rclone config initialisation to init() method
Chris Lu