metadata: move metadata parsing into ExifTool::Metadata.

Move the metadata parsing code from MediaAsset to ExifTool::Metadata so
we can use it outside the context of a MediaAsset, in particular when
dealing with a MediaFile that hasn't been saved to disk yet.

app/models/media_metadata.rb

@@ -23,4 +23,8 @@ class MediaMetadata < ApplicationRecord
   def file=(file_or_path)
     self.metadata = MediaFile.open(file_or_path).metadata
   end
+
+  def metadata
+    ExifTool::Metadata.new(self[:metadata])
+  end
 end