We previously tried to parse the exif datetime as an ISO datetime
string, however the date parse uses colons instead of dashes in exif.
This meant that the modification time was used for every image instead
of the exif date.
Preparing the images list involves reading a directory (which might be
slow on non-local directories) and opening every file to read the exif
data. To avoid having to do this on every request, we do it every 60s in
a background job and only read from the cache on requests.
