StorageManager: remove Post-specific code.

Refactor StorageManager to remove all image URL generation code. Instead
the image URL generation code lives in MediaAsset.

Now StorageManager is only concerned with how to read and write files to
remote storage backends like S3 or SFTP, not with how image URLs should
be generated. This way the file storage code isn't tightly coupled to
posts, so it can be used to store any kind of file, not just images
belonging to posts.

lib/tasks/danbooru.rake

@@ -70,8 +70,7 @@ namespace :danbooru do
 
       MediaMetadata.joins(:media_asset).where(metadata: {}).find_each do |metadata|
         asset = metadata.media_asset
-        file = sm.open(sm.file_path(asset.md5, asset.file_ext, :original))
-        media_file = MediaFile.open(file)
+        media_file = asset.variant(:original).open_file
 
         metadata.update!(metadata: media_file.metadata)
         puts "metadata[id=#{metadata.id}, md5=#{asset.md5}]: #{media_file.metadata.count}"