EXIF date time doesn't always get populated into picture model when importing

[japsu]

After import, some pictures appear without taken_at causing pictures to be in wrong order.

python manage.py refresh_taken_at fixes this, so it is not a matter of not having EXIF creation dates on the files or not being able to read them.

This is probably a race condition of some sort within the media importer. Perhaps someone overwrites the picture model instance with taken_at unset?