Add some data to images to follow metadata updates #122
Description
It would be interesting for owners to know when the metadata associated with an image was at latest updated.
For instance, after an owner exported all the data of a collection, it could be interesting to know a few months later if new corrections or computations have been done and some data need to be updated (without performing a full synchronization).
At minimum, could be good to add a date_updated (or date_metadata_updated), could also be split by type:
date_georef_updated
date_viewshed_updated
date_toponyms_updated
date_metadata_updated (title, description)