* Write media files in chunks
* Test media file writing
* Add iter `ReadDirFiles`
* Remove ImportMediaError, fail fatally instead
Partially reverts commit f8ed4d89ba.
* Compare hashes of media files to be restored
* Improve `MediaCopier::copy()`
* Restore media files atomically with tempfile
* Make downgrade flag an enum
* Remove SchemaVersion::Latest in favour of Option
* Remove sha1 comparison again
* Remove unnecessary repr(u8) (dae)