Add extra image output to ListProcessElements

To support balsac cache, we need to add a filter on ListProcessElements exporting, per element:

• image width
• image height
• image id
• image url
• zone polygon

added 1 deleted label

mentioned in issue tasks#98 (closed)

mentioned in merge request !1278 (merged)

added 3h of time spent at 2021-03-23

closed

mentioned in commit 1359d819