Images : update incorrect date and time using a script

Many images from 828237 to 864267 (~30000 images) have bad date and time, caused by https://github.com/c2corg/v6_ui/issues/1176 and #627
They must be corrected by a script (using alambic ?) :

- Extract date/time stored in the first and the last version of the image doc.
- If the date/time from the first version is equal to the one from the last version :
- Extract EXIF date/time from the original image.
- If the EXIF date/time is different from the last version date/time :
- Edit the image doc to change the date/time with the EXIF date/time.

This algorithm avoids loading the original image if the user has changed the date/time himself (sometime because the APN time was false, or to fixed the wrong extracted value himself).
Also, it avoids changing the date/time if no change is needed.