Backfill license_url field for images where it's null in the meta_data

[krysal]

Problem

As a requirement for #703, we must fill the metadata.license_url field for images without it. At the moment of writing this issue, the number amounts to 97.7 million in the upstream DB.

SELECT COUNT(identifier) FROM image WHERE meta_data->>'license_url' IS NULL;
+----------+
| count    |
|----------|
| 97775299 |
+----------+
SELECT 1
Time: 2732.486s (45 minutes 32 seconds), executed in: 2732.465s (45 minutes 32 seconds)

Description

This potentially could be solved by a one-off DAG that gets the license URL computed from the license and license_version fields. @obulat made one DAG previously for filling one of the cases when the license URL is null, see #1005.

Additional context

See previous work at #1565.

[krysal]

Reopening this since it's still being work out.

[krysal]

The DAG to backfill was triggered yesterday at 14:59:16 UTC and ended successfully after ~8 hours. This is completed. However, we will keep the DAG for some time to see if #4318 keeps repeating.